Due to the way the `render` function was rewritten in eabf971, the audio
variable is always going to be an array of `AudioSample`s, so a check to
see if it is equal to `nothing` is no longer applicable.
As soon as libsndfile has finished reading a file it no longer returns
any data. This can send the `render` method for `FileRenderer` into an
infinite loop. This way if not enough data is available a partial buffer
is returned.
