Add NOT NULL constraint

[lnschroeder]

Closes #42

This also fixes a small spacing issue in the output file:

erDiagram
    person {
        bigint id PK"id of person"
    }

will now render as

erDiagram
    person {
        bigint id PK "id of person"
    }

[KarnerTh]

Thanks, nice work 👍🏼