Fix EZP-23703: Impossible to update only the name of a section

Link: https://jira.ez.no/browse/EZP-23703

## Description
If the identifier is set in `$sectionUpdateStruct` update will be impossible because the update method will think the section already exists.

## Tests
Manual test on the platform UI bundle and added integration tests
you can rather change the first if, this method never returns null, it either throws or returns section.