Return whether the given value is considered empty or not.
This can be anything that might be present in a selector value and thus is typically a string. However, it may be used outside of that purpose so you shouldn't count on it being a string.
Example: an integer or text Fieldtype might not consider a "0" to be empty, whereas a Page reference would.
$bool = $fieldtype->isEmptyValue(Field $field, mixed $value);
API reference based on ProcessWire core version 3.0.137