Unassigned tags can't be renamed

### Expected
Any tag can be renamed

### Observed
If tag is not used in notes, it can't be renamed

### Reproduced
1. Add a new unique tag to any note
2. Make sure tag was created
3. Remove tag from the note
4. Open tags editor and try renaming the recently created tag ~> this doesn't work

Device Make: Samsung
Device Model: A31
Android Version: 10
Simplenote Version: 2.17-rc-1

No source file or test is named. Reproduce the problem in the tags editor using the listed steps, then trace the rename flow for a tag that is no longer attached to a note. Done means an unused tag can be renamed successfully without breaking renaming for tags still used in notes.

Written by the indexing model from the issue text.