Do not mutate the input map in switch closure/transformer factories

[rootvector2]

switchClosure(Map), switchMapClosure(Map) and switchMapTransformer(Map) pulled the default out with map.remove(null) on the caller's map, so the null-key entry vanished from the caller's map as a side effect; spotted while extending the sibling fix in #696 that only touched SwitchTransformer.switchTransformer, they now copy into a LinkedHashMap first.

[garydgregory]

Hello @rootvector2
Thank you for the PR.

Please update the tests here and the new one from #696 to also make sure the result from calling the various switch*() is valid.

Even though we might have other tests that do so, seeing a simple check in these new tests give me more confidence that there are no unintended side-effects.

TY!

[rootvector2]

Done. Each test now executes the built switch*() result and checks it routes the match and the default correctly, and I added the same check to the switchTransformer test from #696.