This allows detecting if it has been set reliably.
0 is a valid payload type.
Bug: webrtc:360058654
Change-Id: Ic3646abe20d0247592145ad27549fa46ddb7ec90
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/362261
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#43016}