fix(rust): ub on empty slices

[Stranger6667]

Hi!

This PR fixes undefined behavior caused by passing a NULL pointer to std::slice::from_raw_parts which manifests in the following test:

#[test]
fn regex_value_test() {
    let result = parse(b"//");
    let node = result.node().as_program_node().unwrap().statements().body().iter().next().unwrap().as_regular_expression_node().unwrap();
    assert_eq!(node.unescaped(), b"");
}

with the following error:

running 1 test
thread 'tests::regex_value_test' panicked at core/src/panicking.rs:223:5:
unsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread caused non-unwinding panic. aborting.
error: test failed, to rerun pass `--lib`

Caused by:
  process didn't exit successfully: `/tmp/prism/rust/target/aarch64-apple-darwin/debug/deps/ruby_prism-3a39e9d2b3dfec9b regex_value_test` (signal: 6, SIGABRT: process abort signal)

The fix itself is a check for NULL before constructing a slice.

[kddnewton]

Thanks!