Fully un-set user-focus filter when empty user is passed

Fix a UI confusion by fully un-setting the user-focus filter if a
empty user value is provided to the `changeFocusModeUser` RPC method.

This prevents a confusing half-set user-focus filter from sticking
around when an instructor returns to viewing all users in grading mode
after focusing on one or more students.

src/sidebar/store/modules/filters.js

@@ -106,15 +106,10 @@ function focusFiltersFromConfig(focusConfig) {
 
 const reducers = {
   CHANGE_FOCUS_MODE_USER: function (state, action) {
-    if (isValidFocusConfig({ user: action.user })) {
     return {
-        focusActive: true,
+      focusActive: isValidFocusConfig({ user: action.user }),
       focusFilters: focusFiltersFromConfig({ user: action.user }),
     };
-    }
-    return {
-      focusActive: false,
-    };
   },
 
   SET_FILTER: function (state, action) {

src/sidebar/store/modules/test/filters-test.js

@@ -36,14 +36,26 @@ describe('sidebar/store/modules/filters', () => {
       //
       // This is the LMS app's way of asking the client to disable focus mode.
       it('deactivates and disables focus if username is undefined', () => {
-        store.toggleFocusMode(true);
+        // Set to a valid user first; this will set and also activate
+        // the filter
+        store.changeFocusModeUser({
+          username: 'testuser',
+          displayName: 'Test User',
+        });
+
+        const firstFilterState = getFiltersState();
+        assert.isTrue(firstFilterState.focusActive);
+        assert.equal(firstFilterState.focusFilters.user.value, 'testuser');
+
+        // Now, emulate the "empty" filter message from the LMS app.
         store.changeFocusModeUser({
           username: undefined,
           displayName: undefined,
         });
-        const filterState = getFiltersState();
-        assert.isFalse(filterState.focusActive);
-        assert.isUndefined(filterState.focusFilters.user);
+
+        const secondFilterState = getFiltersState();
+        assert.isFalse(secondFilterState.focusActive);
+        assert.isUndefined(secondFilterState.focusFilters.user);
       });
     });