Bug 1088377 - Part 1: coerce `url` argument to AVM1's `getURL` to string

So it gets `undefined` or `null` instead of throwing exceptions internally. That's useful ~0% of the time, but matches what Flash does.
Bug 1088377 - Part 2: throw NullPointerError for null/undefined `url` values in AVM2's `navigateToURL`

Much more sensible than what AVM1 does.
Merge pull request #1823 from tschneidereit/master

Bug 1088377 - Fix handling of empty url arguments in AVM1 getURL and AVM2 navigateToURL