GtkApplication: Do not assert sm_proxy != NULL in inhibit functions

GtkApplication: Do not assert sm_proxy != NULL in inhibit functions

Applications have no way of finding out if a session manager proxy was
successfully created in gtk_application_startup_session_dbus(), so it's not
appropriate for certain public GtkApplication functions to be asserting the
presence of a session manager proxy as if it were a programmer error.

This affects:

   gtk_application_inhibit()
   gtk_application_is_inhibited()

If sm_proxy is NULL, the function should just return silently.

In the case of gtk_application_uninhibit(), the application should only be
calling this if it obtained a valid cookie, which implies the presence of a
session manager proxy.  I noted that with a comment.

https://bugzilla.gnome.org/show_bug.cgi?id=701365