The specification states that the field value is `null` if no item is
selected and we didn't handle this case properly. Even though this did
not break the rendering because we always convert the value to an array
and the `includes` check in the display layer would simply not match,
the field value would be `[null]` which is not expected and strange from
an API perspective.
This commit fixes that by ensuring that we return an empty array in
case the field value is `null`. The API therefore still always gives an
array for the field value, but now the code is more specific so that the
value is either an empty array or an array of strings.
12c20772ac