res_pjsip: Ignore empty TLS configuration

When using realtime, fields that are not explicitly set by an
administrator are still presented to sorcery as empty strings. Handle
this case explicitly.

In this particular case, if any of these fields are required for TLS
support, their existence should be validated in the 'apply' handler once
we have a complete transport definition.

ASTERISK-27032 #close
Reported by: seanchann.zhou

Change-Id: Ie3b5fb421977ccdb33e415d4ec52c3fd192601b7