operator: add CrossWithSingleton

A simple, efficient cross-product when the right child
is a singleton
CrossWithSingletonTest: add test with interleaved empty tuple batches
CrossWithSingleton: remove redundant code that checks for empty TBS

Operator actually guarantees we will never get these.
PlanFragmentEncoding: add a helper constructor of
systemtest: add JsonOperatorTest for CrossWithSingleton
Merge pull request #617 from uwescience/cross-with-singleton

operator: add CrossWithSingleton