Empty Multiselection wrong compare and resave object

[betterapp]

@BlackbitDevs

We have a field type:

If this field is empty in previous value we get: NULL
If we return NULL (or even previous value) from tranformation the value is changed to empty array: []

In this case DD always think that field vas changed and resave object - but should not.

DD: 3.9.19

[betterapp]

lib/Pim/Item/FieldMapper/MultiselectMapper.php

the code that return value should be:

        $values = array_values(array_unique($value));

        if (empty($values)) {
            return null;
        }

        return $values;

@BlackbitDevs