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.
It's just the byte-count, but that's an excellent caution, @ctweney : the join alone might kill performance.