Answering a video call (from Android!) does not send an SDP answer when there is no camera

### Steps to reproduce

1. Start a video call from Element Android to a user using Element Web
2. Answer the call on Element Web

### Outcome

#### What did you expect?

* To see the caller's video feed
* *this is what happens when you initiate a call from Element Web*(!)

#### What happened instead?

* The call remains forever in the 'Connecting' state until it eventually times out (the caller gets a message saying the callee didn't answer)
* Firefox's `about:webrtc` shows that no Local SDP Answer is being sent
* Firefox's console is showing lots of activity. It appears to the naïve eye that Element is stuck in a loop trying to request permission to use the camera, but it's automatically failing.

### Operating system

Linux

### Application version

Element version: 1.11.2 Olm version: 3.2.12

### How did you install the app?

Ubuntu PPA

### Homeserver

librepush.net & other internal use homeserver

### Will you send logs?

Yes

N.B. The logs are from the desktop application reproducing the same effect, but my debugging (where I refer to Firefox) has been on a private internal homeserver

Start by reproducing the call in Firefox and inspect about:webrtc alongside the browser console, focusing on the missing Local SDP Answer and repeated camera-permission failures. The issue is done when a video call answered in Element Web completes connection and displays the caller's video even when the callee has no camera.

Written by the indexing model from the issue text.