sip: re-set interface to "default"

Before 1360c775 interface contains
"default", not an empty string. This cause the UDP transport to not
be-ing recycled and changing settings cause the SIP account to not
be re-registered correctly.
Also handle the case of empty interface, anyway it can be considered
as default.

GitLab: #826
Change-Id: I77068e44b7ea93463be2d8dec00b0da87a56370d