Video calls when the macOS device has no webcam

[lissine0]

This issue is related to #152; it describes the case in which your macOS device doesn't have a webcam (I'm testing with a hackintosh without a webcam, but it should be the same with a mac mini)

Description:
If the macOS device has no physical camera:

• The video call button is visible, but if you try to use it, you immediately get "Call failed, It was not possible to establish call" immediately followed by "Call declined"

• If someone videocalls you, Beagle IM rings, but if you accept the call, your contact gets "Call ended: remote busy"
  Expected behaviour: the call should proceed and should be as if you "muted" your camera

• If someone videocalls you, and your xmpp account that is on beagle is also online on another client, you can take the call on the other client [already correct behaviour]

Desktop (please complete the following information):

• OS: macOS 12.7.3
• Version 5.3.5