Annotate optional/nullable columns with `?`

```mmd
erDiagram
    PERSON {
        string firstName
        string? middleName
        string lastName
    }
```
```mermaid
erDiagram
    PERSON {
        string firstName
        string? middleName
        string lastName
    }
```

https://mermaid.js.org/syntax/entityRelationshipDiagram.html#optional-attribute-types-v11-16-0