@staticmethod<EOL><INDENT>def _column_value(orig_val):<DEDENT>
if orig_val is None or orig_val == '<STR_LIT:null>':<EOL><INDENT>return '<STR_LIT>'<EOL><DEDENT>return orig_val<EOL>
Munge a BigQuery column value to what we want to store; currently
just turns ``None`` into the String "unknown".

:param orig_val: original field value
:return: field value we cache/display
:rtype: str