buffer: fix missing null/undefined check

The new implementation of Buffer missed the check for null/undefined as
the first argument to new Buffer(). Reintroduce the check and add test.

Fix: e8734c0 "buffer: implement Uint8Array backed Buffer"
Fix: https://github.com/nodejs/io.js/issues/2194
PR-URL: https://github.com/nodejs/io.js/pull/2195


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>