XWIKI-10057: User's watchlist corrupted when migrating from XE 2.5 to XE 5.3

was: XWIKI-825: Can't delete dangling pages from watchlist after import

 * some watchlists might start with comma
   fix exception thrown when feeding such a string to #unescapeList
 * actually cherry-picked from d8aa8315691e4@master
Merge remote-tracking branch 'origin/stable-5.4.x' into stable-5.4.x