This fixes a bug where Vikunja would not start because the smtp secret was not set. It would show nothing in the logs but `docker inspect` on the host yields
```
"unable to get config from config store: failed to expand templated config cghsr0po6tijn4wuah8zulg48: template: expansion:9:15: executing \"expansion\" at <secret \"smtp_password\">: error calling secret: secret target smtp_password not found",
```
This change fixes that by checking if the mailer is enabled.
