Fixes mailhook missing subject exceptions

This PR should prevent subject-less emails from causing the following MailHook exception:

Exception Encountered

Error ID: 557809

User ID: 
Username: anonymous

URL: https://openstaxtutor.org/mail_hooks/catch

Message: N/A

Exception: #<NoMethodError: undefined method `downcase' for nil:NilClass>

Backtrace:

app/models/mail_hook.rb:109:in `get_mail_fields'
app/models/mail_hook.rb:56:in `matches_for'
app/models/mail_hook.rb:67:in `process'
app/controllers/mail_hooks_controller.rb:12:in `catch'