Content Drip test email doesn't send when Lessons Available email is disabled

[donnapep]

Steps to Reproduce

1. Disable the student Lessons Available email.
2. Scroll down to the Sensei Content Drip section of an existing lesson.
3. Click the Send Test Email button.

What I Expected

I was expecting the email to send.

What Happened Instead

No email was sent.

Describe alternatives you've considered

Hiding or disabling the button if the Lessons Available email is disabled, but I think this could cause confusion as to why the button isn't there / is disabled.

[jondcampbell]

It would at least be better if it didn't provide the user with a success message when the email didn't actually send.