`fieldErrors` is not cleaned up after submit

[marcopiii]

The object fieldErrors: Partial<Record<keyof Values, NonEmptyArray<FieldError>>> returned by useFormo is not properly cleaned up after each submit of the form: if it contained a key foo with some validation error after such error is resolved the key is not removed from the object, instead, its value is set to undefined.

As a consequence using Object.keys(fieldErrors).length > 0 to detect if there is any validation error gives false positives.