fix: contact uri dropped in config load

[ftsef]

The following changes fix an issue where setting the UaSettings.contact_uri had no effect anymore.

In an older version of this library contact_uri was a String

'contact_uri': (Settings src, Settings? dst) {
      dynamic contact_uri = src.contact_uri;
      if (contact_uri == null) return;
      if (contact_uri is String) {
        dynamic uri = Grammar.parse(contact_uri, 'SIP_URI');
        if (uri != -1) {
          dst!.contact_uri = uri;
        }
      }

In the latest version src.contact_uri is an URI?. Therefore checking for a String doesn't make sense anymore and leads to dst.contact_uri being always null.