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