ConfigSet API returns error if untrusted user creates from _default configset

Original title: getConfigMetadata() returns NPE if there is no metadata defined on zk node for configset

While attempting to validate SOLR-16110, I discovered that on main, if you attempt to create a configset via the Configset API, you get a NPE.

Utils.fromJSON doesn't handle getting a null, and the call zkClient.getData(CONFIGS_ZKNODE + "/" + configName, null, null, true) is returning a null byte array.

curl "http://localhost:8983/solr/admin/configs?action=CREATE&name=test&baseName=_default"