Cascaded WMS with client proxy fails when the proxyBaseUrl global.xml setting is set

Because the remote WMS does not get referenced, only the value of [proxyBaseUrl](https://docs.geoserver.org/latest/en/user/configuration/globalsettings.html#proxy-base-url) gets used (in the MapML document's \<map-link rel=image|tile tref="proxyBaseUrl used here...">)