code: public static TableRef valueOf(String schema, String tableName) {
          return new TableRef(null, schema, tableName);
      }
docs: Creates an instance from catalog and table names
  <p/>
  <p>if the schema parameter is an empty String, the catalog or schema will be set to
  <code>null</code>.</p>
  
  @param schema    the table schema
  @param tableName the table name
  @return a <code>TableRef</code> for the table identified by the specified schema and table name.
  @throws IllegalArgumentException if the tableName parameter is null