Check for non-null window in wxRendererNative::GetCheckBoxSize()

Instead of running normally under some platforms and crashing under MSW
(when using themes) when passing NULL to GetCheckBoxSize(), now
consistently assert and return zero size everywhere.

Closes #18241.

interface/wx/renderer.h

@@ -552,7 +552,9 @@ public:
 
     /**
         Returns the size of a check box.
-        The @a win parameter is not used currently and can be @NULL.
+
+        @param win A valid, i.e. non-null, window pointer which is used to get
+            the theme defining the checkbox size under some platforms.
     */
     virtual wxSize GetCheckBoxSize(wxWindow* win) = 0;