always preserve old text when editing a note

[dakkar]

before this change:

• create a note, with text
  • note has a row with the text
  • note_edit doesn't have any matching row
• edit the note, removing all the text
  • the row in note has the new, empty, text
  • note_edit has a matching row, but its oldText is NULL

so the original text is lost

after this change, the row in note_edit always keeps the previous text

Checklist

• [x] Read the contribution guide
• [x] Test working in a local environment
• [ ] (If needed) Add story of storybook
• [ ] (If needed) Update CHANGELOG.md
• [ ] (If possible) Add tests