[Form] Set a child type to text if added to the form without a type.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no, but not because of this PR
| Fixed tickets | #13167
| License       | MIT
| Doc PR        | -

This copies the behaviour of `FormBuilder::create()` to `Form::add()`.

ping @webmozart