Fix assignment from 0-entry Vec: add test (#580)

* Partially revert 8e4ddc62

It was an incomplete fix for handling Vec(0).

* Fix assignment from 0-entry Vec: add test

375e2b6a introduced a regression for bundles
containing zero-entry Vecs.  Until zero-width UInts are supported, the
zero-entry Vecs need to be flattened out before doing asUInt/asTypeOf on
a bundle.  Undoing that commit's replacement of Data.flatten with
Aggregate.getElements is the best interim fix.