Run some peer connection end-to-end tests with an empty audio encoder factory

Specifically, the tests that only use data channels shouldn't need any
audio codec support; by using an audio encoder factory that supports
no codecs, we ensure that this is the case.

(The tests were already using empty *de*coder factories; however, it
was only recently that it became possible to use empty *en*coder
factories as well.)

Bug: webrtc:7529
Change-Id: Ied84283fe88073704a66bc82007b0dfcd7bf377f
Reviewed-on: https://webrtc-review.googlesource.com/c/110726
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25625}