sipcall: answer with audio only to empty offers

Currently, when an empty offer is received, a media offer is
generated with audio, and video if enabled in the account.
Now, the generated offer will include only audio. Both
participants can add video to the call if they wish.
Also some code clean-up

Gitlab: #556

Change-Id: Id51bf44b2b84fd9341de4b67bf53963614088ef8