Handle object-shaped not schemas without crashing

[morgan-coded]

A not: {} schema could reach the complex schema parser and crash because the parser expected complex keyword values to be arrays.

This adds array guards for complex keyword parsing, so non-array values take the same empty-result path as before while array-valued oneOf, anyOf, and allOf behavior stays unchanged. I checked the issue repro against the failing path, then ran install, format, build, and the full test suite with 275 tests passing.

Fixes #1798

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[cubic-dev-ai]

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@morgan-coded hello hello!!!!
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