Low: libcib: Preserve return code in cib__signon_query()

Currently, if we fail to get a query result, we return pcmk_rc_no_input.
This masks the true source of the error, so we should only do this as a
failsafe when the return code is pcmk_rc_ok and there's still no query
result XML.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>