Created by: bpierre
On OS X, the BROWSER environment variable can be set to open to open the default browser.
In this case, the value should not be passed to opn() as a browser option. Doing so would pass it to the open command using the -a parameter and not do anything:
This change passes an empty string in this situation. I added isBrowserValid to make it clearer and to make it easy to add other cases in the future.