res_pjsip/pjsip_configuration: Disregard empty auth values

res_pjsip/pjsip_configuration: Disregard empty auth values

When an endpoint is backed by a non-static conf file backend (such as
the AstDB or Realtime), the 'auth' object may be returned as being an
empty string. Currently, res_pjsip will interpret that as being a valid
auth object, and will attempt to authenticate inbound requests. This
isn't desired; is an auth value is empty (which the name of an auth
object cannot be), we should instead interpret that as being an invalid
auth object and skip it.

ASTERISK-25339 #close

Change-Id: Ic32b0c6eb5575107d5164a8c40099e687cd722c7