Fix ASan null pointer in StartAecDump

Adds a null check to PeerConnectionFactory::StartAecDump to prevent
passing a null FILE* to WebRtcVoiceEngine, which ultimately passes it to
a _Nonnull argument in AudioProcessing::CreateAndAttachAecDump, causing
an ASan/UBSan invalid_null_argument runtime error.

Also updates PeerConnectionFactoryDependenciesTest's
CreatesAudioProcessingWithProvidedBuilder to use a valid temporary file
instead of relying on undefined behavior.

Fixed: webrtc:505372808
Change-Id: I129eb0b650c0dda508e0c99cecaf33892f348bf8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/468260
Reviewed-by: Jeremy Leconte <jleconte@google.com>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47580}