Floss: fix return value of btif_av_get_peer_sep

Some codepaths on deciding whether to start a2dp depends on
the return value of `btif_av_get_peer_sep`.

However, it returns a valid value (sink) even when the peer is not found
(nil), in which case could cause btadapterd to crash at later assert
statements.

Bug: 240641968
Tag: #floss
Test: Build and verify Floss doesn't crash with repro in bug
Change-Id: I0d6839834358fd70d41406364a48ca6152012cba