Haskell's roman numerals doesn't test for Nothing

The current exercise expects numerals :: Integer -> Maybe String which is pretty idiomatic for a function that wouldn’t work on all values of Integer, but the tests only test for Just return values.

It feels weird that the student can implement a partial function wrapped in Just. Can we add Haskell-specific tests for numbers that cannot be rendered in roman numerals, like -1, 0 or 4000?

I think that’s a good idea. The tests are defined here: https://github.com/exercism/haskell/blob/main/exercises/practice/roman-numerals/test/Tests.hs

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Done!