Adding a flatten that doesn't throw out the row if you have an empty bag?

[Jonathan Coveney]

I think this would be nice to have. We could either add a LEFTFLATTEN()
operator, or add a flag to FLATTEN ie FLATTEN({()},true) or something. I
personally prefer a new operator, but know that flatten is baked into pig
so there's that.

I'm willing to do the work, but would like some guidance on what form it
should take, and where to look to make the changes.

Currently you can do

a = group rel by x;
b = foreach a generate FLATTEN((IsEmpty(rel) ? null : rel)) as (val:type);

but this can sometimes result in weird issues, and is just generally clunky
and feels like something pig should do.