Fix display of room notification debug info (#12183)

* Fix display of room notification debug info

 * Add 'one' string because apprently it just displays nothing at all
   if that's missing, rather than erroring or falling back to use the 'other'.
 * Simplify and always display the count: it's debug info so it's best to
   be very explicit anyway, plus simpler code & one less string.

* Update snapshot

test/components/views/dialogs/devtools/__snapshots__/RoomNotifications-test.tsx.snap

@@ -16,6 +16,10 @@ exports[`<RoomNotifications /> should render 1`] = `
             <strong>
               None
             </strong>
+            , count: 
+            <strong>
+              0
+            </strong>
           </span>
         </li>
         <li>