UI: Fix null pointer dereference

strcmp(obs_get_encoder_codec(option), "opus") causes the application to
segfault while loading service if find_encoder(id) returns NULL. Add a
null pointer check to prevent that from occurring.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f