Likert does not allow an "empty()"-ish value.

A likert defined with a value that exists and is valid, but also is considered empty by the "empty()" function will fail validation in. A common value would be "0". I'm not sure if it is possible to define any other valid values, such as the empty string''.

Existing validation:

        if (empty($value[$question_key])) {
          $form_state->setError(...);
        }

Should be:
        if (is_null($value[$question_key])) {
          $form_state->setError(...);
        }

Patch forthcoming.