Consider adding default expression for empty expressions about accumulo-access

[ctubbsii]

I don't think we really need the default expression, since the caller can easily swap out the empty expression it sees in the data with one it is configured to use instead, if it really wanted to, before it calls the evaluator. That's not really something the evaluator needs to do. The same is true about the failing on empty expressions. It'd be weird for the caller to intentionally pass an empty expression to this library, and then expect this library to throw the exception, rather than it throw the exception itself prior to passing it to the library in the first place. I think these features would just bloat the API, without adding much utility to calling code.