PIM-3458 Automatically set next sort order when creating an attribute group

| Q                    | A
| -------------------- | ---
| Bug fix?             | Y
| New feature?         | N
| BC breaks?           | N
| CI currently passes? | ?
| Tests pass?          | Y
| Scenarios pass?      | ?
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | Y
| Fixed tickets        | PIM-3458
| DB schema updated?   | N
| Migration script?    | N
| Doc PR               | ~
in fact, 

`(int) [] -> 0`
`(int) ['test'] -> 1`
`(int) ['test', 'test'] -> 1`