mailing preview unnecessarily requires an active email

when viewing an email, we retrieve token details for the logged in user and pass them when retrieving the mailing object. if the logged in user has no active email (it is on hold) or no email at all (unlikely, but possible), the preview fails with a fatal error (a null object is returned).

url is:
/civicrm/mailing/view?reset=1&id=##

to replicate, login, put your contact's email on hold, and view a mailing using the above url.

although this is a fringe case, I don't think we should require a valid email simply for previewing an email.

here is how we patched it:
https://github.com/nysenate/Bluebird-CRM/commit/03e8171f5d07d4eb0061c64dc9945d99323e546c

if the method is agreeable (I cannot see any potential issues with making those changes), I can generate a PR.