[NTUSER] Remove some extra repaints/redraws (#6827)

Reposition logic and add Flags testing for RgnType == NULLREGION.
This was suggested by @HBelusca and approved by @I_Kill_Bugs

win32ss/user/ntuser/painting.c

@@ -698,6 +698,12 @@ IntInvalidateWindows(PWND Wnd, PREGION Rgn, ULONG Flags)
       RgnType = NULLREGION;
    }
 
+   /* Nothing to paint, just return */
+   if ((RgnType == NULLREGION && (Flags & RDW_INVALIDATE)) || RgnType == ERROR)
+   {
+      return;
+   }
+
    /*
     * Save current state of pending updates
     */