Avoid passing null for the string pointer to JS_EvaluateUCScript.

For empty Vecs, Rust doesn't allocate a backing storage and instead sets the
pointer to null. However, TokenStream::TokenBuf uses null as a poison value,
leading to assertion failures when passing in null into the JS parser.
Merge pull request #91 from Ms2ger/evaluate_script

Avoid passing null for the string pointer to JS_EvaluateUCScript.