FS-6816 [mod_sofia] Set empty callee id if `_undef_`

In some scenarios (e.g. MetaSwitch interop) the `display` field of callee-id should be left empty instead of being overwritten with the number.
As is done in other places, we allow for `_undef_` to mean "leave the field empty".

src/mod/endpoints/mod_sofia/sofia.c

@@ -1173,6 +1173,10 @@ void sofia_send_callee_id(switch_core_session_t *session, const char *name, cons
 		if (zstr(number)) {
 			name = number = "UNKNOWN";
 		}
+
+		if (!zstr(name) && !strcmp(name,"_undef_")) {
+			name = "";
+		}
 	} else {
 		if (zstr(name)) {
 			name = caller_profile->callee_id_name;
@@ -1189,6 +1193,10 @@ void sofia_send_callee_id(switch_core_session_t *session, const char *name, cons
 		if (zstr(number)) {
 			number = caller_profile->destination_number;
 		}
+
+		if (!zstr(name) && !strcmp(name,"_undef_")) {
+			name = "";
+		}
 	}
 
 	if ((uuid = switch_channel_get_partner_uuid(channel)) && (session_b = switch_core_session_locate(uuid))) {