Fix error getting absent person

The PersonQueryResolver returns Optional.absent() when a Person's
source is disabled for the request's application. The subsequent
Optional.get() therefore throws an ISE. This avoids the get() by
simply testing isPresent(). It can't distinguish between a non-
existent person and an invisible person at this point so the return
code is 404.
Merge pull request #343 from atlasapi/fix-unavailable-people

Fix error getting absent person