Use better safety in rectangular grid

- Checking `DrawSize != Vector2.Zero` is too specific. It could also
  crash on zero-height-but-non-zero-width, or
  zero-width-but-non-zero-height.

- Take the `gridCache.Validate()` call out of the zero checks, because
  even if the width or height are zero, not generating anything is valid
  and there is no reason to validate every frame until `gridCache` gets
  invalidated again.

osu.Game/Screens/Edit/Compose/Components/RectangularPositionSnapGrid.cs

@@ -55,11 +55,10 @@ namespace osu.Game.Screens.Edit.Compose.Components
             {
                 ClearInternal();
 
-                if (DrawSize != Vector2.Zero)
-                {
+                if (DrawWidth > 0 && DrawHeight > 0)
                     createContent();
-                    gridCache.Validate();
-                }
+
+                gridCache.Validate();
             }
         }