[Backport staging] use correct default for notification silencing (#9389)

Co-authored-by: Kerry <kerrya@element.io>

test/utils/notifications-test.ts

@@ -47,8 +47,8 @@ describe('notifications', () => {
     });
 
     describe('localNotificationsAreSilenced', () => {
-        it('defaults to true when no setting exists', () => {
-            expect(localNotificationsAreSilenced(mockClient)).toBeTruthy();
+        it('defaults to false when no setting exists', () => {
+            expect(localNotificationsAreSilenced(mockClient)).toBeFalsy();
         });
         it('checks the persisted value', () => {
             mockClient.setAccountData(accountDataEventKey, { is_silenced: true });