Subcategories cannot be dropped into non-expandable subcategories

### Shopware Version

6.7.0.0

### Affected area / extension

Platform(Default)

### Actual behaviour

You can’t drag a subcategory into another subcategory unless that subcategory already contains at least one child. Only then it becomes expandable and accepts other subcategories via drag and drop.

### Expected behaviour

Empty subcategories should render as expandable drop targets to allow inserting subcategories even if they don’t have children yet.

### How to reproduce

1. open the category tree
2. create a new empty subcategory
3. try to move via drag and drop one category into the new created subcategory.

### Definition of Done

- [ ] An automated test covers the fix.
- [ ] Integration/E2E testing in staging is done.
- [ ] Changelog markdown file created or updated in /changelog/_unreleased directory.
- [ ] Developer documentation written/updated.
- [ ] End user and/or developer documentation written or updated with clarification.
- [ ] Regression test created to ensure bug does not reappear, and automated regression tests pass.
- [ ] Code changes resolve the reported bug without introducing new issues.