Allow nil to be used for param functions

[zombiezen]

From @zombiezen on August 26, 2015 0:19

Quite often, it's useful to have interface methods that have no parameters. It's somewhat unruly to write out:

promise := obj.NiladicFunc(ctx, func(p schema.Object_niladicFunc_Params) error {
  return nil
})

instead of the more concise:

promise := obj.NiladicFunc(ctx, nil)

Copied from original issue: zombiezen/go-capnproto#9