gio: Handle NULL cached properties in NetworkManager monitor

g_dbus_proxy_get_cached_property() and
g_dbus_proxy_get_cached_property_names() can both return NULL if the
property cache is empty. Avoid a crash if this situation arises (which
it looks like it could, from reading the code) by gracefully bailing out
on NULL return values.

Coverity issues: #1257044, #1257045

https://bugzilla.gnome.org/show_bug.cgi?id=741229