I was experiencing choppy video playback on a Meet call and thought I'd capture some logs but pressing "Start Debug Mode" in about:webrtc never gave me a filename with the log.

(Relatedly, I was speaking with EKR who was able to get a filename (I think he said it was /tmp/webrtc.log) but in his case the file was empty.)

What platform was this on?

Ok, it looks like you found this when trying to debug bug 1736434, where you mention that you were running an M1 Mac, so I'm assuming that's the platform for this one too.

So, this pref is being set to the empty string by default:

https://searchfox.org/mozilla-central/rev/36aa22c7ea92bd3cf7910774004fff7e63341cf5/modules/libpref/init/all.js#448

I don't see any other place in the tree where we are setting this. We probably need to give this a sensible default value, otherwise these buttons on about:webrtc are probably not going to help us with other people's bug reports.