HMaster#getClusterSchema() could return null and cause NPE

HMaster#getClusterSchema() has a lot of usage, for example:

void assignTableToGroup(TableDescriptor desc) throws IOException {
  String groupName =
      master.getClusterSchema().getNamespace(desc.getTableName().getNamespaceAsString())
                .getConfigurationValue(RSGroupInfo.NAMESPACE_DESC_PROP_GROUP);
  ...
}

getClusterSchema() might return null when cluster schema service is not ready yet. Actually, it won't be ready until HMaster#initClusterSchemaService() is finished. See this comment for details.

We need to add protections, like adding null check before calling getClusterSchema().getXXX(), or in HMaster#getClusterSchema(), add a loop to wait until this.clusterSchemaService is not null.