[Form] Fixed patched forms to be valid even if children are not submitted

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8111
| License       | MIT
| Doc PR        | -

#8362 was reverted because it introduces a failing test that is caused by a regression. This PR takes the alternative approach that

* unsubmitted fields in the PATCH request remain unsubmitted
* `isValid()` ignores unsubmitted children
* `mapFormsToData()` ignores unsubmitted children

In my opinion this is a more proper solution than #8362.