Subject is blank for some emails when title is too long

[donnapep]

Steps to Reproduce

1. Create a lesson named "This is a Super Long Lesson Title For Testing Purposes".
2. Enable the teacher's "Lesson Completed" email.
3. Complete the lesson.
4. Check the email that was sent.

What I Expected

The email subject to be "admin completed This is a Super Long Lesson Title For Testing Purposes".

What Happened Instead

The email subject is empty. Note that the same thing happens for almost all other emails if the dynamic title is too long.