Actually reset the `findResultsCount` label, in addition to hiding it, when no matches are found

Currently we're *only* hiding the label, but not actually resetting it until a new match is found.
Obviously it's being hidden, but it seems that it really ought to be completely reset as well (since e.g. `PDFFindBar.reset` won't technically reset *all* state otherwise).

web/pdf_find_bar.js

@@ -151,16 +151,16 @@ class PDFFindBar {
       return; // No UI control is provided.
     }
 
-    // If there are no matches, hide the counter.
     if (!matchCount) {
+      // If there are no matches, hide and reset the counter.
       this.findResultsCount.classList.add('hidden');
-      return;
-    }
-
-    // Create and show the match counter.
+      this.findResultsCount.textContent = '';
+    } else {
+      // Update and show the match counter.
       this.findResultsCount.textContent = matchCount.toLocaleString();
       this.findResultsCount.classList.remove('hidden');
     }
+  }
 
   open() {
     if (!this.opened) {