> What is the initial `''` for?

When testing for unique prefixes, I need to have a string to compare to. Prepending the `''` means that I don't have to special-case the handling of `Object.keys(o)[0]`. There may be cleaner ways to do it, of course.

I used `''` because it needs to be a string that comes before every other key in `o`. I could have used `'-'` or `'--'` too, in this case, but `''` is more general, and that always makes me happy.

Note that this code also assumes that `''` is not already part of `Object.keys(o)`. This should never happen as long as `this.opts_` is never directly accessed. Actually, come to think of it, I'm pretty sure this code would work correctly in the `''` case also. Doesn't matter for getopt, but good to know should this code get transplanted somewhere else.