presto-phoenix/src/main/java/io/prestosql/plugin/phoenix/MetadataUtil.java/MetadataUtil/toPrestoSchemaName
class MetadataUtil:    public static Optional<String> toPrestoSchemaName(Optional<String> phoenixSchemaName)
    {
        return phoenixSchemaName.map(schemaName -> schemaName.isEmpty() ? DEFAULT_SCHEMA : schemaName);
    }