Audioflinger: Do not intercept if no audio

This was not efficient and leaded to an assert in obtainBuffer.
I'm not sure which condition can lead to a getNextBuffer of size 0,
but it has been observed and is not forbidden by getNextBuffer
documentation.

Test: atest android.media.cts.AudioPlaybackCaptureTest#testCaptureMediaUsage
Bug: 111453086
Change-Id: I5accf7c1d488ff4686272588329bab71d64f67cd
Signed-off-by: Kevin Rocard <krocard@google.com>