[Tags] Deleting tag via renaming leaves an empty tag in notes

### Expected
When tag is deleted, it's deleted completely

### Observed
There's a way to delete a tag this way: start renaming a tag, erase all text and hit space or 'done' key. When doing so, the tag instances in notes are replaced with an empty tag.

Note: this is the same in 4.30, so it's not a result of recent changes in tags editor.

### Reproduced
See video.
1. Create a new note
2. Create several tags in this note
3. Go to tags editor
4. Start renaming one of the tags, erase all text, tap space or 'done' key on keyboard

https://user-images.githubusercontent.com/73365754/105983884-0896c880-60a2-11eb-84e3-91abf0a755ae.MP4

Make|Model|iOS Version|App Version
-|-|-|-
iPhone|XR|14.3|4.31

Start by reproducing the issue in the iOS tags editor using the listed steps: rename a tag, erase its text, and submit with space or Done. Trace how the editor handles the empty name and verify that deleting a tag removes its instances rather than leaving empty tags in notes.

Written by the indexing model from the issue text.