startPluginEdit pre-filled any empty field with its descriptor default,
treating an explicitly cleared field (stored "") the same as one never set
(undefined). So a saved-blank OpenSky bounding box reappeared as the
default 50.5,3.2,53.7,7.3 on reopen (and would be re-persisted on save).
Only inject a default when the field has never been stored (undefined), so
intentional blanks stay blank.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
