Send `null` for string-like columns that don't allow empty strings when cell is cleared

[pavish]

Description

• Open or create a table with a column that allows string input that performs some validation.
  • Date & Time
  • Date
  • Email
  • URI etc.,
• Let's say we have a URI column.
• Edit a cell and enter some string., eg., "test".
• This will result in an error. That's expected.
• Now edit the cell again and remove everything clearing the cell.
• This will result in an error again, when it shouldn't.
• That's because the frontend sends the patch request with an empty string "".
• (The user can still right click and set to null, but that's not always obvious in this situation).
• For such columns that have custom validation and do not allow empty strings, the frontend should send null instead of empty string "".