Return NULL for empty ParametersNode

[haldun]

Fixes #961

[kddnewton]

A couple of thoughts on this.

1. I think the case where we have invalid parameters should still add parameters to the list. For example def foo(A), you'd still expect there to be a node in the list of required parameters, even though it's incorrect. That way there's a place to attach the error message and formatters/linters have something to point to.
2. Instead of allocating the parameters node and then immediately deallocating, I would much prefer that we simply never allocate in the first place. I think this is probably going to be solved by the first point above, but there may be other cases where we need to look forward instead.

[haldun]

@kddnewton About your first point, is c1fe0ec close to what you suggest?

About 2, I think I don't know how to do it otherwise. Don't we need a place in memory to collect the parameters?

[kddnewton]

@haldun I've come around on this. Could you rebase and make sure the tests pass?

[haldun]

@kddnewton Done in f82ef87