LeAudio: Fix running configuration matcher on nullptr

Codec Manager should not run the verifier on a nullptr and just return
no configuration.

Bug: 332476151
Test: atest --host bluetooth_le_audio_codec_manager_test --no-bazel-mode
Flag: EXEMPT; Trivial fix - unit tested
Change-Id: I4569d8547698399bf614cd4797fd91b20ff169f6