CLC-4064 - Roster Photos Feature doesn't print nicely

https://jira.ets.berkeley.edu/jira/browse/CLC-4064

When there is no picture available, the API shouldn't return any. On the front-end we check for it and if it doesn't exist, we show the no photo available.
Let's keep a close eye on this -- get_enrolled_students is used in a lot of places, so it's possible that the additional join to the photo table will have an effect on performance. I can't look at the db right now, but how large is the photo_bytes column data? Is that just an integer count of bytes?