[Form] Fixed ValidatorTypeGuesser to guess unmapped properties not to be required

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6645
| License       | MIT
| Doc PR        | -

This PR is WIP and should not yet be merged. It is one of two alternative solutions to #6645. The other alternative is to stepwisely set the default value of the `required` option to `false`, as described in #6645.