[SMF] smf_gy_send_ccr: Allow NULL xact

The xact may well be NULL, eg. when tearin down the session
(send_ccr_termination_req_gx_gy_s6b()) because OCS rejected an update:
Hence there's no GTP xact originating the tear down, aka e->gtp-xact
passed to the function is NULL.

smf_gx_send_ccr() is already handling this case properly, contrary to smf_gf_send_ccr().

src/smf/gy-path.c

@@ -612,7 +612,6 @@ void smf_gy_send_ccr(smf_sess_t *sess, void *xact,
     const char *service_context_id = "32251@3gpp.org";
     uint32_t timestamp, req_slot;
 
-    ogs_assert(xact);
     ogs_assert(sess);
 
     ogs_assert(sess->ipv4 || sess->ipv6);