Handled null cursor correctly in wxQt SetCursor()

Make passing invalid wxCursor object to SetCursor() work and reset any
previously specified cursor.

Closes https://github.com/wxWidgets/wxWidgets/pull/1197

src/qt/window.cpp

@@ -518,7 +518,10 @@ bool wxWindowQt::SetCursor( const wxCursor &cursor )
     if (!wxWindowBase::SetCursor(cursor))
         return false;
 
-    GetHandle()->setCursor(cursor.GetHandle());
+    if ( cursor.IsOk() )
+        GetHandle()->setCursor(cursor.GetHandle());
+    else
+        GetHandle()->unsetCursor();
 
     return true;
 }