btif_sock: prevent crash from logging NULL UUIDs

It is valid to pass a NULL UUID into btsock_connect. Therefore, this
condition must be checked before converting it to string for logging.

Bug: 328763360
Bug: 328764473
Test: m -j
Test: Pass CTSV CoC tests
Flag: EXEMPT, logging changes
Change-Id: I70a7549983593c38aa1d90c892f4adcd6ce24494