Fix wxWindow::setBackgroundColour(wxNullColour) on OS X

Commit 91aa6ba36e introduced a
regression causing this to crash. We need to validate that the color
is valid before attempting to call OSXGetNSColor; if it is not, we
should clear any previously-set background color.

Closes #18470.

Closes https://github.com/wxWidgets/wxWidgets/pull/1503

src/osx/cocoa/window.mm

@@ -3177,7 +3177,8 @@ void wxWidgetCocoaImpl::SetBackgroundColour( const wxColour &col )
             wxTopLevelWindow* toplevel = wxDynamicCast(peer,wxTopLevelWindow);
 
             if ( toplevel == NULL || toplevel->GetShape().IsEmpty() )
-                [targetView setBackgroundColor: col.OSXGetNSColor()];
+                [targetView setBackgroundColor:
+                        col.IsOk() ? col.OSXGetNSColor() : nil];
         }
     }
 }