Fix media writer failing to capture silent video

If the user has denied microphone access, the media writer would never write any sample buffers so it's status would be AVAssetWriterStatusUnknown when ```finishWritingWithCompletionHandler:``` was called. This meant the ```vision:capturedVideo:error:``` delegate method would never be called.

Since it's valid to capture video without audio, this commit sets the ```audioReady``` state to YES if the user has denied microphone access. ```captureOutput:didOutputSampleBuffer:fromConnection:``` can then start passing video data to the writer without waiting for the audio output to be setup (which will never happen if the user has denied mic access)