vcl: fix utterly mysterious error C2064 term does not evaluate

... to a function taking 0 arguments with MSVC 2013.

Change-Id: I73744bfe9def70d00e6e489f7064472cdb64b113

vcl/source/control/button.cxx

@@ -1544,7 +1544,7 @@ bool PushButton::PreNotify( NotifyEvent& rNEvt )
 
 void PushButton::Toggle()
 {
-    ImplCallEventListenersAndHandler( VCLEVENT_PUSHBUTTON_TOGGLE, NULL );
+    ImplCallEventListenersAndHandler( VCLEVENT_PUSHBUTTON_TOGGLE, nullptr );
 }
 
 void PushButton::SetSymbol( SymbolType eSymbol )