Handle quirky gnome_keyring_find_password_sync() behavior

Looks like gnome_keyring_find_password_sync() is pretty busted - it
returns GNOME_KEYRING_RESULT_OK when returning a NULL password. This
is probably due to the libgnome-keyring transition. Therefore, double
check that we actually get a password back.

See https://bugzilla.redhat.com/show_bug.cgi?id=558678 for more
details.