jni: allow null as preshared key

[an-p]

Allows setting the arg_preshard_key argument for create_new_tunnel to null in order to create a tunnel without a preshared key.

Fixes #228.

[Noah-Kennedy]

@an-p the macos CI tests can sometimes fail spuriously. Looks like this is happening here. Could you push an empty commit to restart CI?

[an-p]

This doesn't seem to help. macOS arm64 tests keep timing out, probably because of this:

docker: unknown server OS: .
See 'docker run --help'.

[Noah-Kennedy]

@an-p I haven't had time to investigate, but this usually works, and only sometimes fails. I'd try one more time. If that fails I'm gonna remove this test from CI for the time being.

[Noah-Kennedy]

@an-p so I have good news and bad news. The good news is that we fixed the CI issues on the master branch, and all you need to do here is merge in those changes. The bad news is that there are some mean merge conflicts here, as we just split the crate into a workspace, which should have been done long ago.

[an-p]

I have rebased my PR on the latest master branch. All tests passed.