[Form] fix deprecated usage and clarify constructor defaults for number formatter

By using the real default one can see the actual value without having to look into the implementation what NULL stands for

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | n/a