How to make displaylength parameter optional in SQL server

ALTER procedure [dbo].[spGetRetailerDealerServiceCenterList]
(
        @DisplayLength   int           = NULL,
		@DisplayStart    int           = NULL,
		@SortCol         int           = NULL,
		@SortDir         nvarchar(10)  = NULL,
		@Search          nvarchar(255) = NULL,
 	    @FromDate        VARCHAR(20)   = null,
		@ToDate          VARCHAR(20)   = null,
		@Action          char(1)       = null
)
as
begin
        Set @FromDate = ISNULL (@FromDate,'')
		Set @ToDate   = ISNULL (@ToDate,'')

		--if(@FromDate != '' and @ToDate != '')
		--begin		
		--   set @DisplayLength = 100
		--end 
		
		Declare @FirstRec int, @LastRec int,@OffsetValue int,@PagingSize int
		Set @FirstRec = @DisplayStart;
		Set @LastRec = @DisplayStart + @DisplayLength;

		--print   @FromDate
		--print   @ToDate

if(@Action='A')
begin

	
			With CTE_RetailerList as
							(
						Select  Distinct ROW_NUMBER() over (order by
        
								case when (@SortCol = 0 and @SortDir='asc')
								 then DM.DealerId
								end asc,
								case when (@SortCol = 0 and @SortDir='desc')
								 then  DM.DealerId
								end desc,
        
								case when (@SortCol = 1 and @SortDir='asc')
									 then DM.DealerName
								end asc,
								case when (@SortCol = 1 and @SortDir='desc')
									then   DM.DealerName
								end desc,

								case when (@SortCol = 2 and @SortDir='asc')
									then DM.Code
								end asc,
								case when (@SortCol = 2 and @SortDir='desc')
									then DM.Code
								end desc,

								case when (@SortCol = 3 and @SortDir='asc')
									then DM.[Address]
								end asc,
								case when (@SortCol = 3 and @SortDir='desc')
									then DM.[Address]
								end desc,

								case when (@SortCol = 4 and @SortDir='asc')
									then DM.ContactNo
								end asc,
								case when (@SortCol = 4 and @SortDir='desc')
									then DM.ContactNo
								end desc,		

								case when (@SortCol = 5 and @SortDir='asc')
									then DM.EmailId
								end asc,
								case when (@SortCol = 5 and @SortDir='desc')
									then DM.EmailId
								end desc,
		
								case when (@SortCol = 6 and @SortDir='asc')
									then ZM.ZONE_NM
								end asc,
								case when (@SortCol = 6 and @SortDir='desc')
									then ZM.ZONE_NM
								end desc,						
		
								case when (@SortCol = 7 and @SortDir='asc')
									then SM.STATE_NM
								end asc,
								case when (@SortCol = 7 and @SortDir='desc')
									then SM.STATE_NM
								end desc,
		
								case when (@SortCol = 8 and @SortDir='asc')
									then ADM.District_NM
								end asc,
								case when (@SortCol = 8 and @SortDir='desc')
									then ADM.District_NM
								end desc,

								case when (@SortCol = 9 and @SortDir='asc')
									then CM.CITY_NM
								end asc,
								case when (@SortCol = 9 and @SortDir='desc')
									then CM.CITY_NM
								end desc,
								case when (@SortCol = 10 and @SortDir='asc')
									then BM.BRANCH_Name
								end asc,
								case when (@SortCol = 10 and @SortDir='desc')
									then BM.BRANCH_Name
								end desc,

								case when (@SortCol = 11 and @SortDir='asc')
									then DM.CreatedDate
								end asc,
								case when (@SortCol = 11 and @SortDir='desc')
									then DM.CreatedDate
								end desc

								)
								    as RowNum,

								    COUNT(*) over() as TotalCount,
						
									ISNULL	(DM.DealerName,'NA') as DEALERNAME,
									ISNULL	(DM.DealerId,0 )as DEALERID, 			
									ISNULL	(DM.Code,0) as DEALERCODE,				
									 											
									ISNULL	(DM.[Address],'NA') as [ADDRESS],			
									ISNULL	(DM.ContactNo,'NA') as CONTACTNO,			
									ISNULL	(DM.EmailId,'NA')as EMAIL ,				
									ISNULL	(ZM.ZONE_NM,'NA') as ZONENAME,			
									ISNULL	(SM.STATE_NM ,'NA')as STATENAME,			
									ISNULL	(ADM.District_NM,'NA' )as DISTRICTNAME,	
									ISNULL	(CM.CITY_NM,'NA') as CITYNAME,			
									ISNULL	(BM.BRANCH_Name,'NA') as BRANCHNAME,		
								    ISNULL(convert(varchar(20), DM.CreatedDate,103)+' '+convert(varchar(20),convert(time, DM.CreatedDate),100),0)CREATEDDATE,
								    ISNULL(DATENAME(MONTH, DM.CreatedDate),0)Months,
								    ISNULL(CONVERT(VARCHAR(10), CAST( DM.CreatedDate AS TIME), 0),0) times
					
								
									FROM [dbo].[Dealer_Master] DM WITH(NOLOCK)
	
									INNER JOIN	dbo.ZoneMaster ZM WITH(NOLOCK) on ZM.ZONE_ID=DM.ZoneId
									INNER JOIN	[dbo].state_master SM WITH(NOLOCK) on  SM.STATE_ID=DM.StateId
									INNER JOIN	[dbo].[A_DistrictMaster] ADM WITH(NOLOCK) on  ADM.District_ID=DM.DistrictId
									INNER JOIN	[dbo].[A_CITY_MASTER] CM WITH(NOLOCK) on CM.CITY_ID=DM.CityId
									INNER JOIN	[dbo].branch_master BM WITH(NOLOCK) on BM.BRANCH_ID=DM.BranchId											
									
									
									where ((DM.DealerName IS NOT NULL) 

															     
								    AND 
									(
									CONVERT(VARCHAR,DM.CreatedDate,112) >= CONVERT(VARCHAR,CONVERT(DATETIME,@FromDate),112) OR CONVERT(VARCHAR,CONVERT(DATETIME,@FromDate),112) IS NULL 
									AND   CONVERT(VARCHAR,DM.CreatedDate,112) <=  CONVERT(VARCHAR,CONVERT(DATETIME,@ToDate),112)  OR CONVERT(VARCHAR,CONVERT(DATETIME,@ToDate),112)  IS NULL )
									
									AND (  @Search IS NULL
							
										 Or DM.DealerId          like '%' + @Search + '%'
										 Or DM.DealerName		 like '%' + @Search + '%'
										 Or DM.Code				 like '%' + @Search + '%'
										
										 Or DM.[address]         like '%' + @Search + '%'	
										 Or DM.contactno         like '%' + @Search + '%'
										 Or DM.EmailId           like '%' + @Search + '%'
										 Or SM.STATE_NM          like '%' + @Search + '%'
										 Or ADM.District_NM      like '%' + @Search + '%'
										
										 Or CM.CITY_NM           like '%' + @Search + '%'
										 Or BM.BRANCH_Name       like '%' + @Search + '%'
										
										 )	 
							))
							Select *
							from CTE_RetailerList
							where RowNum > @FirstRec and RowNum <= @LastRec	 

							order by  CreatedDate  desc		
end

end