🍒 applets/clipboard: Show appropriate placeholder message with no search results

The clipboard applet would show "Clipboard is empty" when no matches were found, instead of a more appropriate message. This fixes it and has the applet show "No matches found" and the edit-none icon when no results are found in the searchbar, as per the original intended behaviour.

BUG: 486300 FIXED-IN: 6.0.5

(cherry picked from commit e73a835e)

aed7b739 BUG: 486300 924632d4 Merge branch plasma-workspace:master into fix-klipper-empty-message