Only one of sdpMLineIndex or sdpMid should be required in Candidate

[zecakeh]

Link to problem area: https://spec.matrix.org/v1.2/client-server-api/#mcallcandidates

Issue
According to the WebRTC spec, both sdpMid and sdpMLineIndex are nullable, but one of those two values must be set:

If both the sdpMid and sdpMLineIndex members of candidateInitDict are null, throw a TypeError. (source)

Currently in the spec they are both marked as required.

Expected behaviour
Both fields should be nullable, explaining that at least one of the two is required.

In practice, both the matrix-js-sdk and Element Android check that at least one of those is set, respecting the WebRTC spec, which makes this a spec bug rather than requiring an MSC.

[richvdh]

Still the case in https://spec.matrix.org/v1.6/client-server-api/#mcallcandidates.

@dbkr: can you confirm this?

[zecakeh]

Fixed with #1742.