Handling of false in then or else

[Zash]

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I noticed that our JSON Schema implementation is doing a simple check for the not, then and else keywords, meaning a schema like {"not":false} would be treated exactly like {}. I imagine this should matter in some conditions but I can't quite get my head around the double negatives needed. This does not seem to have been caught by the draft2020-12 tests as of 7950d9e and I see no changes since then to not.json or `if-then-else.json