Handle the user's email address being null

Service users (ie ssh access keys) don't have a valid email, so this breaks
the email validation.

Just skip all name checks for service users - access key users are
automated systems, and the 'name' is generally meaningless too (its the
SSH key comment)

Fixes #21
Merge pull request #22 from bbaetz/fix-access-keys

Handle the user's email address being null