media attribute: update media source only if set

When a new media list is provided (during a media change request
for instance), the media source and source type will be updated
only if set in the new media attribute list. This will prevent unwanted
overwriting of media source and source type.
This patch will fix the ut_media_negotiation unit test.

Gitlab: #576

Change-Id: If6f994def1e113265eeff0cce93fe5d314dffd5d