[Form] Allow NumberToLocalizedStringTransformer::reverseTransform to accept int/float

| Q             | A
| ------------- | ---
| Bug fix?      | not sure
| New feature?  | not sure
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

When adding a form type `integer` with a `empty_data => 0` (aka with a int or float value) the form field will never validate correctly because the `NumberToLocalizedStringTransformer` will throw a `TransformationFailedException` because the value is not a string.

This really PR fixes this problem by allowing int and float types as values in `reverseTransform`. Because the string only behavior was enforced by the transformer tests this is a BC change. (Personally I think that this is not a BC break that would cause problems).

Another possible fix is to add a normalizer for the `empty_data` option and transform a int or float to a LocalizedString but that looks/feels like a bad patch job.

I hope this PR will have less people going through there validation schema just to figure out that it's the `NumberToLocalizedStringTransformer` causing the problem.