callmodel: don't turn video on when accepting a call in audio

The function which is responsible for muting the camera when accepting
a call in audio assumes that the call's mediaList has already been
initialized, but this wasn't actually the case. This caused a bug where
the 'Accept in audio' button behaved exactly like the 'Accept in video'
button.

GitLab: #1621
Change-Id: I26251f51862cf5cd9b6d4daaf15270943c0e3c4e