Hide empty wxCheckBoxes in wxGTK to fix display problem

Focus rectangle was still drawn around the label when it was empty with some
themes, so hide it completely to ensure this doesn't happen.

Closes #17443.

src/gtk/checkbox.cpp

@@ -204,6 +204,13 @@ void wxCheckBox::SetLabel( const wxString& label )
 {
     wxCHECK_RET( m_widgetLabel != NULL, wxT("invalid checkbox") );
 
+    // If we don't hide the empty label, in some themes a focus rectangle is
+    // still drawn around it and this looks out of place.
+    if ( label.empty() )
+        gtk_widget_hide(m_widgetLabel);
+    else
+        gtk_widget_show(m_widgetLabel);
+
     // save the label inside m_label in case user calls GetLabel() later
     wxControl::SetLabel(label);