When syncing highlights between Readest and KOReader, the `note` field
was forced to an empty string (`""`) for annotations and bookmarks
without notes. KOReader's native annotations omit the field entirely
when no note exists, so the empty string caused KOReader to treat
every synced highlight as having a (blank) note. Apply the same
omission in both push and pull directions.
Co-authored-by: Claude <noreply@anthropic.com>
Bailey Jennings