SIP Account - Handle and test send/receive of empty offers (no SDP)

Handle send and receive of INVITEs with no media offer (no SDP). Empty offers are already supported in incoming incoming INVITEs. We need to be able to send empty offers as well and handle the media offer sent in the 200 OK answer. Unit tests must be added to validate handling of empty INVITEs in incoming and outgoing calls.