### Steps to reproduce

1. Login as an admin

2. Go to Administration > Custom field > Work packages

3. Open an existing hierarchy custom field Begin of the insertionwith sub itemsEnd of the insertion or create Begin of the deletion a newEnd of the deletion one

4. Add sub-items

5. Save Begin of the deletion the 1stEnd of the deletion sub-item Begin of the insertionAEnd of the insertion without a short name

6. Create Begin of the insertionsub-item BEnd of the insertion Begin of the deletion another sub itemEnd of the deletion without a short name and save it Begin of the insertion

7. Edit sub-item B, remove the short name and save itEnd of the insertion


### What is the buggy behavior?

* The empty short name value is considered a duplicate

* Sub item cannot be saved because it doesn't have a short name


### What is the expected behavior?

1. Several sub items can be saved even without short names


Begin of the insertion### Note

it doesn't happen on sub item creation, only when the sub item is edited

End of the insertion ### Screenshots and other files

<img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/248215/content">

### Environment information

**OpenProject version**

_15.1.0_

**Browser**

* [ ] Chrome
* [x] Firefox
* [ ] Safari
* [ ] Mobile Safari
* [ ] Other (please specify)

**Operating System**

* [x] Windows
* [ ] Mac OS X
* [ ] Mobile iOS
* [ ] Mobile Android
* [ ] Linux (please specify distro)
* [ ] Chrome OS
* [ ] Other (please specify)