Make index() sas 3.3 compatible

This PR makes `index()` compatible with ruby sass 3.3.0 by returning null rather than `false` if the index doesn't exists.

Fixes https://github.com/sass/libsass/issues/512. Spec added https://github.com/sass/sass-spec/pull/62, https://github.com/sass/sass-spec/pull/85, https://github.com/sass/sass-spec/pull/86