Check if menu item is null before adding assistant new badge

It crashes because there's no assistant in AOSP Contacts.

Bug: 32915307

Test: manual - verified that there's no crash in opening AOSP Contacts
               and that the new badge works well on GoogleContacts.

Change-Id: I3cc14da0c9e50fe0b2f4840645ded8c2563fbc65
1 file changed