Keep empty folders in the tree list

What does this MR do?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60860

Before	After
Empty-folders-before	Empty-folders-after

In essence, we keep an empty folder in the tree list after removing the last file from it. It is convenient if user needs to re-upload a file to the same folder.

NOTE This MR doesn't keep the empty folder after reload. Since there no blobs in the folders anymore, we do not get information about those from the server. Hence, if we need to make empty folders stay permanent in the list, we should involve backend but I think the current solution is sufficient for now.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Closes #60860 (closed)