Don't allow empty list constraints in Conduit calls

Summary:
Ref T11473. If you write a method like get_stuff(ids) and then call it with an empty list of IDs, you can end up passing an empty constraint to Conduit.

If you run a *.search method with such a constraint, like this one:

{
  "ids": []
}

...we have three possible beahviors:

1. Treat it like the user passed no constraint (basically, ignore the constraint).
2. Respect the constraint (return no results).
3. Error.

Currently, we do (1). However, this is pretty confusing and I think clearly the worst option, since it means get_stuff(array()) in client code will often tend to return a ton of results.

We could do (2) instead, but this is also sort of confusing (it may not be obvious why nothing matched, even though it's an application bug) and I think most reasonable client code should be doing an if ($ids) test: this test makes clients a little more complicated, but they can save a network call, and I think they often need to do this test anyway (for example, to show the user a different message).

This implements (3), and just considers these to be errors: this is the least tricky behavior, it's consistent with what we do in PHP, makes fairly good sense, and the only cost for this is that client code may need to be slightly more complex, but this slightly more complex code is usually better code.

Test Plan: Ran Conduit *.search queries with "ids":[] and "phids":[], got sensible errors instead of runaway result sets.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11473

Differential Revision: https://secure.phabricator.com/D16396