Unable to unselect Default Folder

Steps to reproduce

• go to Account settings -> Default folders
• open selection for any option (e.g. "Drafts are saved in:")
• press Backspace in order to invalidate selection
• the combobox now says "Select...", and implies that the default folder has been unselected
• reload the page, and go back to Account settings -> Default folders

Expected behavior

The blanked default folder type is unselected.

Actual behavior

I find the previous value assigned to the manipolated default folder.

Mail app version

5.10

Nextcloud version

No response

Mailserver or service

No response

Operating system

No response

PHP engine version

None

Nextcloud memory caching

No response

Web server

None

Database

None

Additional info

In those conditions, this.mainStore.patchAccount issues a call to /apps/mail/api/accounts/1 with an empty payload, which doesn't implies any actual change on backend.

But the real question is: is it possible at all to unassigned a default folder?

If yes: some fix is required to submit a "empty this selection" to the backend.
If not: frontend must impede to unselect that selection.

I've found this issue working on #13318, which still includes the same problem (as I didn't know how to handle this scenario).

Start in the Default folders settings flow and trace the mainStore.patchAccount call to /apps/mail/api/accounts/1 when a selection is cleared. Determine whether an empty default-folder value can be persisted; done means either the cleared value survives reload or the UI prevents unselecting it, with the behavior from #13318 considered.