Possibility to search for null in backend grids

[digitalpianism]

Description (*)

Possibility for backend users to search for NULL values in the column filters.

Expected behavior (*)

When the entered value is "NULL", filters the values for NULL MySQL value.

Benefits

Helps the users to filter in some specific cases (custom attributes, custom grids etc...).

Additional information

I already have the code to implement this feature, will do a PR soon.