subdomains: Fix some implicit uses of "" for the root subdomain.

These are just instances that jumped out at me while working on the
subdomains code, mostly while grepping for get_subdomain call sites.
I haven't attempted a comprehensive search, and there are likely
still others left.

zerver/views/auth.py

@@ -660,7 +660,7 @@ def get_auth_backends_data(request):
         realm = Realm.objects.get(string_id=subdomain)
     except Realm.DoesNotExist:
         # If not the root subdomain, this is an error
-        if subdomain != "":
+        if subdomain != Realm.SUBDOMAIN_FOR_ROOT_DOMAIN:
             raise JsonableError(_("Invalid subdomain"))
         # With the root subdomain, it's an error or not depending
         # whether ROOT_DOMAIN_LANDING_PAGE (which indicates whether