Implement Series.from_list of null type

[lkarthee]

Fix for #806

Support create Series.from_list([nil, nil])

Even though tests pass - DataFrame.concat_rows may fail and Series.concat will fail if one of dtypes is :null (irrespective of this fix) - need to address it later.

sn = Series.from_list([nil, nil, nil], dtype: :null)
s2 = Series.from_list([4.0, 5.0, 6.4])
Series.concat([s2, sn]) # will fail - it is valid in polars