Implement guessRequired based on nullable in mapping

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [yes]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes|no]
| Fixed tickets | n.a.
| License       | MIT
| Doc PR        | n.a.