Prevent pushing falsey email keys to permissions stack.

Per [72591652](https://www.pivotaltracker.com/story/show/72591652).  Error message was traced to how one of the email values in the request was blank (turned out to be that of diana@reciprocitylabs.com, who is attacked to the program in the bug), which in turn was due to how the email lookup for that account returned an empty string.  We might want to have more robust handling for cases like this, but for now it should be enough to remove such emails from the request stack.