Avoid crash in wxBitmap::GetImage() for invalid bitmaps

Just return NULL from this (wxOSX private, in spite of not using a
port-specific prefix) method.

This fixes crash in wxButton::SetBitmapXXX(wxNullBitmap), as shown by
the new test case which used to crash but doesn't do it any longer.

Closes #19257.

include/wx/osx/bitmap.h

@@ -204,6 +204,7 @@ public:
     // returns a CGImageRef which must released after usage with CGImageRelease
     CGImageRef CreateCGImage() const ;
 
+    // returns nil for invalid bitmap
     WXImage GetImage() const;
 #if wxOSX_USE_COCOA
     // returns an autoreleased version of the image