The null case of a nullable-poiner enum might not be nullary.

Cases like `Either<@int,()>` have a null case with at most one value but
a nonzero number of fields; if we misreport this, then bad things can
happen inside of, for example, pattern matching.

Closes #6117.
auto merge of #6134 : jld/rust/issue-6117, r=catamorphism

Cases like `Either<@int,()>` have a null case with at most one value but
a nonzero number of fields; if we misreport this, then bad things can
happen inside of, for example, pattern matching.

Closes #6117.