Check if the video has an audio track before fallback

This would break transcoding for videos without an audio track as the codec checking would be null referencing.

Jellyfin.Api/Helpers/StreamingHelpers.cs

@@ -236,7 +236,7 @@ public static class StreamingHelpers
                 }
             }
 
-            if (!EncodingHelper.IsCopyCodec(state.OutputAudioCodec) && string.Equals(state.AudioStream.Codec, state.OutputAudioCodec, StringComparison.OrdinalIgnoreCase) && state.OutputAudioBitrate.HasValue)
+            if (state.AudioStream is not null && !EncodingHelper.IsCopyCodec(state.OutputAudioCodec) && string.Equals(state.AudioStream.Codec, state.OutputAudioCodec, StringComparison.OrdinalIgnoreCase) && state.OutputAudioBitrate.HasValue)
             {
                 state.OutputAudioCodec = state.SupportedAudioCodecs.Where(c => !EncodingHelper.LosslessAudioCodecs.Contains(c)).FirstOrDefault(mediaEncoder.CanEncodeToAudioCodec);
             }