Do not explode if recipient has no email address

# References
* https://www.pivotaltracker.com/n/projects/880854/stories/80401056

# Notes
* This technically should not happen anymore (on new users) because user now has a validation on email address.  However, this is probably good practice to not explode if the recipient does not have an email address.  Also, this change makes it extremely easy to use on other mailers (one liner)

--- JN + AC
Yes, `mail` in this case is an instance of a `Mail::Message`.  Otherwise ... ouch.