The answer is: You never know. There are lots of optimizations going on in modern JavaScript engines. One example is dead code elimination, which skips code that does not influence the end result.
There was a quite interesting controversy about this feature in IE9:
http://digitizor.com/2010/11/17/internet-explorer-9-caught-cheating-in-sunspider-benchmark/
But why would you want to run an empty block over and over anyway?
If you want the JavaScript interpreter to simply wait try this answers:
What is the JavaScript version of sleep()?
Sleep in Javascript - delay between actions