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
minor #9488 [Form] fix deprecated usage and clarify constructor defaults for number formatter (Tobion)

This PR was submitted for the 2.4-dev branch but it was merged into the 2.4 branch instead (closes #9488).

Discussion
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[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

Commits
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5a25989 [Form] fix deprecated usage and clarify constructor defaults for number formatter