Remove unnecessary value check

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see if it is equal to `nothing` is no longer applicable.

src/sndfile.jl

@@ -162,10 +162,6 @@ function render(node::FileRenderer, device_input::AudioBuf, info::DeviceInfo)
         audio = hcat(audio, read_audio)
     end
 
-    if audio == Nothing
-        return AudioSample[]
-    end
-
     # if the file is stereo, mix the two channels together
     if node.file.sfinfo.channels == 2
         return (audio[1, :] / 2) + (audio[2, :] / 2)