make sure all of the needed fields are present

There has been a fix (587142501419279ee405fbf2c28d4913dcd0e9cd) to make sure older versions of elasticsearch that don't return the 'fs' field when permissions don't allow don't break the collector, but in 0.20 ES's behavior seems to be returning `{..., "fs": { "timestamp": xxxxxxx, data: [] }, ...}` instead.

I added a better ward to this to avoid index exceptions