Switch to explicit checking against None in constructor.

In [rrule.py:371](https://github.com/dateutil/dateutil/blob/master/dateutil/rrule.py#L371), it seems to me that the existence of the `byweekno`, `byyearday` and `bymonthday` parameters are being checked just by their boolean truth values, rather than checking them against `None` (the default value for each of them).

I'm not sure if there's some reason it is done this way, but it can cause a problem when the sequence you want to pass is a numpy array rather than one of the standard sequence types, because the truth value of numpy arrays is not well-defined, causing an error to be thrown.