send_custom_email: Never email users who haven't agreed to ToS.

The `create_user` API and data import tools can result in our having
active users in the database who haven't intentionally created a Zulip
account or agreed to the ToS; we should never email such users.

The check for `TOS_VERSION is not None` is necessary for the
development environment, which has `TERMS_OF_SERVICE` set but not
`TOS_VERSION`.

It's likely that we will want this check in other places as well.

zerver/management/commands/send_custom_email.py

@@ -1,6 +1,8 @@
 from argparse import ArgumentParser
 from typing import Any
 
+from django.conf import settings
+
 from zerver.lib.management import CommandError, ZulipBaseCommand
 from zerver.lib.send_email import send_custom_email
 from zerver.models import Realm, UserProfile
@@ -86,6 +88,9 @@ class Command(ZulipBaseCommand):
                     )
                 raise error
 
+        # Only email users who've agreed to the terms of service.
+        if settings.TOS_VERSION is not None:
+            users = users.exclude(tos_version=None)
         send_custom_email(users, options)
 
         if options["dry_run"]: