MT#19795 rtc_sessions deprecate network tag

by adding the default value. Not specifying the network tag anymore, will now
have no negative effect, but specifying it won't break things either. Thus
making it effectively backwards-compatible according to the new company-guideline
supporting rollbacks.
This description in ngcp-schema is purely cosmetical.

Change-Id: I59f1be63fa037271840957b08551b65261152a0b

lib/NGCP/Schema/Result/rtc_session.pm

@@ -38,6 +38,7 @@ __PACKAGE__->add_columns(
     data_type => "varchar",
     is_nullable => 0,
     size => 45,
+    default_value => "",
   },
 );