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Fix #20590 with another application of mkHsContextMaybe

Ryan Scott requested to merge wip/T20590 into master

We were always converting empty GADT contexts to Just [] in GHC.ThToHs, which caused the pretty-printer to always print them as () => .... This is easily fixed by using the mkHsContextMaybe function when converting GADT contexts so that empty contexts are turned to Nothing. This is in the same tradition established in commit 4c87a3d1.

In the process of fixing this, I discovered that the Cxt argument to mkHsContextMaybe is completely unnecessary, as we can just as well check if the LHsContext GhcPs argument is empty.

Fixes #20590 (closed).

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