Change Declaration of Var 'j' to None (#921)

Since `j` is redefined before it is used, it makes more sense to declare it with the value `None` instead of `1`.

This fixes a [warning from lgtm](https://lgtm.com/projects/g/TheAlgorithms/Python/snapshot/66c4afbd0f28f9989f35ddbeb5c9263390c5d192/files/other/primelib.py?sort=name&dir=ASC&mode=heatmap)