Email templates without a 'no-reply' address set fail when used in actions

Abridged traceback from a `DeclinedInstructorsAction`:

```
File "/webapps/amy.carpentries.org/repo/amy/autoemails/actions.py", line 205, in __call__ return self.email.send(fail_silently=False)
...
File "/webapps/amy.carpentries.org/repo/.venv/lib/python3.11/site-packages/anymail/utils.py", line 159, in parse_address_list
raise AnymailInvalidAddress(errmsg)
anymail.exceptions.AnymailInvalidAddress: Invalid email address '' parsed from '' in `reply_to`.```
```
We have a few templates with no 'reply-to' address - `declined-instructor-signups`, `profile-update`, `consents-new` - but the latter two aren't used.

Start with amy/autoemails/actions.py at line 205 and reproduce the DeclinedInstructorsAction failure. Inspect the email templates declined-instructor-signups, profile-update, and consents-new to compare their reply-to settings. Done means templates without a reply-to address no longer fail when used in actions, while configured addresses continue to work.

Written by the indexing model from the issue text.