Remove early abort for NULL values in uiprivTableModelSetCellValue.

We are changing the value, update the row!

There are several uses for setting NULL values, IMO all abusing the
API for different causes. Fix those API abuses instead:

- This will cause one unneeded redraw when clicking on table column
  buttons as they signal a click through setting NULL. Possibly use
  a dedicated callback handler for that instead?
- This will fix resetting colors to default, as this is signaled
  through a NULL value.

common/tablemodel.c

@@ -42,12 +42,6 @@ void uiprivTableModelSetCellValue(uiTableModel *m, int row, int column, const ui
 	mh = uiprivTableModelHandler(m);
 	(*(mh->SetCellValue))(mh, m, row, column, value);
 
-	// Abort redraw for button columns which signal being clicked
-	// by setting NULL.
-	// TODO check if NULL is never a valid value otherwise
-	if (value == NULL)
-		return;
-
 	uiTableModelRowChanged(m, row);
 }