Reset store annotations on `FrameSyncService.hoverAnnotations(null)`

Ensure that hovered annotations are cleared in the store when `null` is
passed to `FrameSyncService.hoverAnnotations`.

src/sidebar/services/frame-sync.ts

@@ -530,14 +530,14 @@ export class FrameSyncService {
   hoverAnnotation(ann: Annotation | null) {
     this._pendingHoverTag = null;
 
+    const tags = ann ? [ann.$tag] : [];
+    this._store.hoverAnnotations(tags);
+
     if (!ann) {
       this._guestRPC.forEach(rpc => rpc.call('hoverAnnotations', []));
       return;
     }
 
-    const tags = ann ? [ann.$tag] : [];
-    this._store.hoverAnnotations(tags);
-
     // If annotation is not currently anchored in a guest, schedule hover for
     // when annotation is anchored. This can happen if an annotation is for a
     // different chapter of an EPUB than the currently loaded one. See notes in

src/sidebar/services/test/frame-sync-test.js

@@ -926,6 +926,9 @@ describe('FrameSyncService', () => {
         fakeStore.hoverAnnotations,
         sinon.match.array.deepEquals([fixtures.ann.$tag])
       );
+
+      frameSync.hoverAnnotation(null);
+      assert.calledWith(fakeStore.hoverAnnotations, []);
     });
 
     it('focuses the associated highlights in the guest', () => {