Don't add an empty element in some cases

This patch does apply on beta6, but throws a notice:

Warning: Undefined variable $max in Drupal\multivalue_form_element\Element\MultiValue::processMultiValue() (line 234 of modules/contrib/multivalue_form_element/src/Element/MultiValue.php).

Same thing with dev-version.

Im seeing the same notice, although the Delta feels a bit odd in its original state, doesnt this mean every element _weight array has the same delta? I think we can reintroduce the variable to prevent the notices, but not sure I fully understand the original use.