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

src/sip/sipaccount.cpp

@@ -1959,7 +1959,7 @@ SIPAccount::createBindingAddress()
     const auto& conf = config();
 
     IpAddr ret = conf.bindAddress.empty()
-                     ? (conf.interface == ip_utils::DEFAULT_INTERFACE
+                     ? (conf.interface == ip_utils::DEFAULT_INTERFACE || conf.interface.empty()
                             ? ip_utils::getAnyHostAddr(family)
                             : ip_utils::getInterfaceAddr(getLocalInterface(), family))
                      : IpAddr(conf.bindAddress, family);

src/sip/sipaccount_config.h

@@ -45,7 +45,7 @@ struct SipAccountConfig : public SipAccountBaseConfig {
     /**
      * interface name on which this account is bound
      */
-    std::string interface;
+    std::string interface {ip_utils::DEFAULT_INTERFACE};
 
     /**
      * Determine if STUN public address resolution is required to register this account. In this