[SIP] Video not displayed if initially blank and only received after some time

Noticed this while working with https://github.com/nextcloud/spreed/pull/13486 on the SIP bridge.

Currently the video unmute event is triggered if more than 2000 bytes are received within a second of the stream starting:
https://github.com/nextcloud/spreed/blob/d63691af4228767d37249d4649f04049305e230f/src/utils/webrtc/webrtc.js#L1231-L1241

If less than 2000 bytes (but more than 0) are received, the video is assumed as "disabled" and further checks are disabled:
https://github.com/nextcloud/spreed/blob/d63691af4228767d37249d4649f04049305e230f/src/utils/webrtc/webrtc.js#L1313

While this doesn't happen all the time, when it happens, only a datachannel unmute event will make the video appear in the callview:
https://github.com/nextcloud/spreed/blob/d63691af4228767d37249d4649f04049305e230f/src/utils/webrtc/webrtc.js#L1575

This is a problem for SIP video which might be empty / black at the beginning and only start after some time, and datachannels are not supported by the SIP bridge.

Adding here for tracking. Maybe @danxuliu has an idea if there is a simple way to support this case without having to implement datachannels in the SIP bridge.

Start in src/utils/webrtc/webrtc.js at the linked sections around lines 1231, 1313, and 1575. Trace how initial byte counts disable further checks and how the datachannel unmute event makes video appear. Done means delayed, initially blank SIP video can appear without relying on datachannels.