Handle the fact that several pua generated documents could exist.

Sometimes, pua_usrloc generates several XML documents for the same
presentity. It can happen if a refresh fails.

We should ignore all of them when federating presence, and not only the
last one.
Handle the case where rpid:activities exist but has no children.

It was not take into account when caching the federated presence state.
When it happens, we should rely on the standard PIDF document to
determine the global presence state as the RPIDF document gives no
information.