Parses a timerange argument and always returns non-None timerange.

def _ParseTimeRange(self, timerange):
    """Parses a timerange argument and always returns non-None timerange."""
    if timerange is None:
      timerange = (None, None)

    from_time, to_time = timerange
    if not from_time:
      from_time = rdfvalue.RDFDatetime().FromSecondsSinceEpoch(0)

    if not to_time:
      to_time = rdfvalue.RDFDatetime().FromSecondsSinceEpoch(sys.maxsize)

    return (from_time, to_time)