Haskell-http-generator - Make endpoints which don't return anything yield NoContent

[ivanbakel]

Fixes #9901

The haskell-http-client generator tries to generate a polymorphic return type for endpoints which don't return anything in the success case, but still produce content in other cases. This means that these endpoints hit a decoding error in the success case, because there is no content to decode.

This changes the behaviour so that endpoints that don't return anything are always generated as returning NoContent, and never try to decode the response. This change is based on a similar one for the haskell-servant generator, which can be found at #9830, which resolved a similar issue for that generator.

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[wing328]

cc @jonschoning