Only add media bundle if offer SDP contains any media fields, useful for data only connections

[fakuivan]

Disclaimer: I'm not a WebRTC expert, in fact I only know the basics to get a simple connection up, event then this bug almost made me quit using this library, this is why I'm reporting it.

In the cases where a data only peer connection needs to be established aiortc responds to an SDP offer with an answer containing an empty BUNDLE group, this empty group is supposed to be filled with some m=s, but it never does since no media is declared and Chrome complains failing to set up the connection, for firefox this works fine.

This is the error that pops up when adding the SDP from aiortc to the browser:

Error: Failed to execute 'setRemoteDescription' on 'RTCPeerConnection': Failed to set remote answer sdp: A BUNDLE group contains a MID='' matching no m= section.

Here's the SDP produced by the browser:

{
    "type": "offer",
    "sdp": "v=0\r\no=- 449907951519833443 2 IN IP4 127.0.0.1\r\ns=-\r\nt=0 0\r\na=extmap-allow-mixed\r\na=msid-semantic: WMS\r\n"
}

and here's the response from aiortc:

{
    "type": "answer",
    "sdp": "v=0\r\no=- 3900077871 3900077871 IN IP4 0.0.0.0\r\ns=-\r\nt=0 0\r\na=group:BUNDLE \r\na=msid-semantic:WMS *\r\n"
}

My solution was to change the following lines here:

aiortc/src/aiortc/rtcpeerconnection.py

Lines 558 to 561 in 9f14474

	bundle = sdp.GroupDescription(semantic="BUNDLE", items=[])
	for media in description.media:
	bundle.items.append(media.rtp.muxId)
	description.group.append(bundle)

to this:

if len(description.media) > 0:
    bundle = sdp.GroupDescription(semantic="BUNDLE", items=[]) 
    for media in description.media: 
        bundle.items.append(media.rtp.muxId) 
    description.group.append(bundle) 

This is the answer after the fix:

{
    "type":"answer",
    "sdp":"v=0\r\no=- 3900078276 3900078276 IN IP4 0.0.0.0\r\ns=-\r\nt=0 0\r\na=msid-semantic:WMS *\r\n"
}

Is this fix correct? Would this pass as a pr?

[github-actions]

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