notnull(): Series {
    return this.where(null, (a, b) => a !== b);
  }
Returns a boolean same-sized Series indicating if the values are not null

pandas equivalent: [Series.notnull](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.notnull.html)

@returns {Series}

@example
const ds = new Series([1, 2, null, null, 4]);

// Returns Series([true, true, false, false, true])
ds.notnull();