fix(downgrader): stop emitting nullable without type in 3.1 to 3.0

[dinwwwh]

The 3.1 → 3.0 downgrader no longer puts nullable: true on schemas that have no type. Null-only schemas now come out as { enum: [null] }. As a result, the Pydantic/FastAPI pattern anyOf: [X, { type: 'null' }] produces schemas that Ajv 8 can compile.

Fixes

• type: 'null' and const: null no longer produce nullable without type. OpenAPI 3.0 ignores that combination, and Ajv 8 refuses to compile it ("nullable" cannot be used without "type").
• const: null alongside several types no longer adds nullable at the top level, where there is no type, so these schemas compile too.
• { const: null, type: 'string' } now accepts no value, like the source schema. Before, it accepted null.

Schemas whose type lists null alongside another type are unchanged: they still get type plus nullable: true.

Testing

• Every converted case compiles in Ajv 8 and accepts the same values as the 3.1 source.
• Updated the null-only and const: null expectations, and added cases for the anyOf pattern, a multi-type null const, and a null const with a non-null type.
• The README's schema mapping table is updated to match.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[pullfrog]

✅ No new issues found.

Reviewed changes

• Null-only schemas no longer emit nullable without type — the rest.length === 0 branch in applyTypes drops out.nullable = true, so { type: 'null' } now converts to { enum: [null] } instead of the Ajv-invalid { enum: [null], nullable: true }.
• const: null no longer forces nullable — convertConst is reduced to emitting the single-value enum, which also fixes { const: null, type: 'string' }: it previously accepted null and now correctly accepts nothing.
• Tests and README — updated the null-only and const: null expectations, added the anyOf: [X, { type: 'null' }] FastAPI pattern, the multi-type null const, and the contradicting-type cases; synced the mapping table.

I independently reproduced the Ajv 8.20.0 behaviour against the pnpm-store copy: { nullable: true } fails to compile with "nullable" cannot be used without "type", while { enum: [null] }, { type: 'string', enum: [null] }, and anyOf: [{ type: 'string' }, { enum: [null] }] all compile and accept exactly the values the 3.1 sources do. pnpm test (353 passing), pnpm lint, and pnpm type:check are all green.

One non-blocking note: the repo has no Ajv dependency, so the semantic-equivalence checks described in the PR body are not captured by the in-repo suite (@seriousme/openapi-schema-validator only validates structure). The specific regression is still pinned by the toEqual expectations, so nothing needs to change here.

  ^{｜ View workflow run ｜ Using DeepSeek Flash (default — pick a model for stronger reviews) ｜ 𝕏}