The check for whether the AVP exists in the ULA was wrong, resulting in
the MME refusing to proceed without this AVP. However, it is not a
mandatory AVP.
At least in version 11.8.0 Release 11 of 3GPP TS 29.272,
Subscribed-Periodic-RAU-TAU-Timer is only permitted inside
Subscription-Data. Before this patch, the MME expects it outside, i.e.
on the "main" level of the DIAMETER AVP tree.
Closes: #238