Avoid empty rectangular tooltip datagrid cell? Any suggestion

I am not exactly sure what you mean by an empty cell so I cannot give an exact solution to your less than precise problem statement but you have at least 3 options to explore:

1> Use a FallBack binding option:
You can specify to show a certain data item when the normal data item is not available:
WPF Binding FallbackValue set to Binding - Stack Overflow[^]

- Either use the FallbackValue as shown in the question section of the linked page or use the alternative binding via

PriorityBinding

shown in the answer section.

2> Use a converter: You can always use a statndard IValueConverter to do essantially the same default value display logic as in 1>. The Difference is that the converter is a C#/VB.Net piece of code that gives you way may options to integrate different and larger scenarios should you see many different cases and want to react to them in a different fashion.

3> An

IMultiValueConverter

gives you most flexibility and options towards binding from multiple sources and converting values into a common format. Here is an example that I just typed out (have not compiled it) - it takes a string and a boolean value (eg IsEnabled) as input and converts it into a string.

[ValueConversion(typeof(string), typeof(string))]
public class ItemTypeDisplayNameToTextConverter : IMultiValueConverter
{
  public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
  {
    if (values == null)
        return Binding.DoNothing;

    if (values.Length != 2)
        return Binding.DoNothing;

    var item = values[0] as string;

    if (item == null)
        return Binding.DoNothing;

    if (values[1] is bool == false)
        return Binding.DoNothing;

    bool isEnabled = (bool)values[1];

	if (isEnabled == false)           // The cell was not enabled
	  return string.Empty;
	
	if (string.IsNUllOrEmtpy(item))  // There was no string being successfully bound
	  return string.Empty;

    return item;
}

  object[] IMultiValueConverter.ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
  {
      throw new NotImplementedException();
  }
}

You can download a sample solution, set a breakpoint in the converter, and start the demo if you want to see this working in-situ:
InplaceEditBoxLib/ItemTypeDisplayNameToTextConverter.cs at master · Dirkster99/InplaceEditBoxLib · GitHub[^]

I hope this helps. Let me know if you were able to solve your problem or if you see a different problem.