QuickCheck random generator produces empty tuples.

[jurgenvinju]

Describe the bug

Empty tuples exist in vallang, but not in Rascal. Either we fix that or we make sure the random
generator does not produce empty tuples. We can not write code that matches them or produces them, etc.
The result is that tests fail without a way of even filtering them before the test is run.

[jurgenvinju]

I added <> to Expression and Pattern and that she's the job. Of course we still can't type in a type for tuple[] but that's for another issue.

[PaulKlint]

So this can be closed?

[jurgenvinju]

We still have to add support for empty tuple expressions and patterns to the checker and the compiler