Fixes bug causing silent videos to crash (no audio track)

If not accounted for, silent videos (0 audio tracks) would crash on this line:
```
self.audioOutput = [AVAssetReaderAudioMixOutput assetReaderAudioMixOutputWithAudioTracks:audioTracks audioSettings:nil];
```

This implements a simple fix by skipping the audioOutput code when no tracks are present in the video file

```
NSArray *audioTracks = [self.asset tracksWithMediaType:AVMediaTypeAudio];
  if (audioTracks.count > 0) {
    //..do the audio stuffs
  }
```