def value_to_none(config_val, evar):
if not config_val:<EOL><INDENT>return None<EOL><DEDENT>return config_val<EOL>
Given a value that evaluates to a boolean False, return None.

:param str config_val: The env var value.
:param EnvironmentVariable evar: The EVar object we are validating
    a value for.
:rtype: str or None
:return: Either the non-False value or None.