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.
The speedup provided is not very noticeable on my machine (~0.5%) but could help on slower hardware like the Raspberry Pis.