[applets/clipboard] Refine placeholder message when searching

Now it will say "No matches" when there are no search results, and
"Clipboard is empty" when the clipboard actually is empty.

BUG: 437386
FIXED-IN: 5.23

applets/clipboard/contents/ui/Menu.qml

@@ -63,7 +63,7 @@ PlasmaExtras.ScrollArea {
             width: parent.width - (PlasmaCore.Units.largeSpacing * 4)
 
             visible: menuListView.count === 0
-            text: i18n("Clipboard is empty")
+            text: model.filterRegExp.length  > 0 ? i18n("No matches") : i18n("Clipboard is empty")
         }
     }
 }