Make callback in RunEventLoop() optional

[luminosuslight]

The callback is not always required, i.e. in the normal node Start() method.

node/src/node_lib.h

Lines 158 to 160 in 1959f89

	NODE_EXTERN void RunEventLoop(
	const std::function<void()> & callback,
	UvLoopBehavior behavior = UvLoopBehavior::RUN_NOWAIT);