'A Filter producing True for values where this Factor has missing data.
Equivalent to self.isnan() when ``self.dtype`` is float64.
Otherwise equivalent to ``self.eq(self.missing_value)``.
Returns
filter : zipline.pipeline.filters.Filter'

def isnull(self):

    if (self.dtype == float64_dtype):
        return self.isnan()
    else:
        return NullFilter(self)
