It might happen that a UE has no CU-UP (e.g., never requested a PDU
session). When triggering a release, we previously and implicitly
associated a CU-UP in that case. That is not good, and confusing.

This commit adds a function to look up if the UE has an associated
CU-UP. We only send a release if it is the case.

The function get_existing_cuup_for_ue() now instead verifies that a
CU-UP exist, and does not implicitly create an association (which might
be unwanted, see above).