build: only define NODE_V8_OPTIONS if not empty

Previously, V8::SetFlagsFromString was called on every launch even if
NODE_V8_OPTIONS was an empty string. This patch only defines
NODE_V8_OPTIONS if node_v8_options is not an empty string.

PR-URL: https://github.com/nodejs/node/pull/2532


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rod Vagg <rod@vagg.org>