[gtk] Fix checking for for py_widget
[gtk] Fix checking for for py_widget

py_widget can't be NULL and Py_None at the same time. Thanks to jessevdk
for pointing at this on #clutter.

pmarti/pyclutter,GNOME/pyclutter,pmarti/pyclutter,GNOME/pyclutter,pmarti/pyclutter,pmarti/pyclutter