src: only set v8 flags if argc > 1

ParseArgs sets the first element of v8_args to argv[0], so v8_argc will
always be at least 1. This change only calls
V8::SetFlagsFromCommandLine if v8_argc > 1, leading to an additional
startup improvement of ~5%.

PR-URL: https://github.com/nodejs/node/pull/2646


Reviewed-By: trevnorris - Trevor Norris <trev.norris@gmail.com>