[BEAM-3527] Adding a comment for ZERO element of DistributionResult.

[pabloem]

I've decided to not rename the ZERO, because I get the feeling that IDENTITY might be even less clear (though I'm open to having my mind changed:) ). I've only added a comment to clarify its purpose.
cc: @bjchambers
cc: @clairemcginty

[robertwb]

+1 to changing your mind. Another alternative would be EMPTY. I though ZERO was the distribution with nothing but zeros in it.

[clairemcginty]

@robertwb EMPTY could be good as it matches what GaugeResult has -- although, still not a perfect analogue as EmptyGaugeResult just returns a value of -1.

[pabloem]

hmm how about we go with IDENTITY_ELEMENT ?

[clairemcginty]

Sounds good to me!

[pabloem]

r: @kennknowles
getting another reviewer

[kennknowles]

SGTM LGTM