Fixes two issues:
1. lang-javascript doesn't support `executable` with a `null` `vars`
parameters. The parameter is quite nullable.
2. reindex didn't support script engines who's `unwrap` method wasn't
a noop. This didn't come up for lang-groovy or lang-painless because
both of those `unwrap`s were noops. lang-javascript copys all maps that
it `unwrap`s.
This adds fairly low level unit tests for these fixes but dosen't add
an integration test that makes sure that reindex and lang-javascript
play well together. That'd make backporting this difficult and would
add a fairly significant amount of time to the build for a fairly rare
interaction. Hopefully the unit tests will be enough.
e07e5d66fa