Railo null support

I made some minor changes to be able to use TestBox for Railo with null support enabled. Nothing that would break compatibility when null support is off (or nonexistent).

When a function is called with named arguments, Railo passes null for the arguments that are not passed. With null support enabled this causes problems.

Thanks for considering this fix, and I'd be happy to hear your feedback.
to be clear, I opened the original ticket and was going to submit a pull request, but this one was already submitted by https://github.com/neoneo and this is his code.

But to answer your question, yes, in the case of the url scope it _probably_ wouldn't matter if you left it as structKeyExists, but isNull will handle the same thing and will also protect against the variable being set directly to null - which is much more likely in the case of arguments than url ( and indeed is the primary reason that this PR is necessary).

At the end of the day, isNull will do what you want and also give you greater protection against both the variable not being defined but also explicitly being set to null.