Reset wxStaticBitmap image when passed empty bitmap in wxGTK

This makes the behaviour consistent with the other ports.

Closes #17420.

src/gtk/statbmp.cpp

@@ -59,16 +59,16 @@ void wxStaticBitmap::SetBitmap( const wxBitmap &bitmap )
 {
     m_bitmap = bitmap;
 
-    if (m_bitmap.IsOk())
-    {
     // always use pixbuf, because pixmap mask does not
     // work with disabled images in some themes
+    if (m_bitmap.IsOk())
         gtk_image_set_from_pixbuf(GTK_IMAGE(m_widget), m_bitmap.GetPixbuf());
+    else
+        gtk_image_set_from_pixbuf(GTK_IMAGE(m_widget), NULL);
 
     InvalidateBestSize();
     SetSize(GetBestSize());
 }
-}
 
 // static
 wxVisualAttributes