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.

src/osx/core/bitmap.cpp

@@ -951,7 +951,7 @@ bool wxBitmap::Create(CGContextRef bitmapcontext)
 
 WXImage wxBitmap::GetImage() const
 {
-    return GetBitmapData()->GetImage();
+    return IsOk() ? GetBitmapData()->GetImage() : NULL;
 }
 
 wxBitmap wxBitmap::GetSubBitmap(const wxRect &rect) const