Fixes an issue with NullAudioPoller calling the mixer every 10 ms when
no call is ongoing.
Bug: b/142775365
Change-Id: I77eeddadaf08b358cce2b389c70e4f2baf1d5627
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/157176
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Commit-Queue: Gustaf Ullberg <gustaf@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29508}