[zod-openapi] Inconsistent and incorrect handling of z.null() between OAS 3.0 and 3.1

[hexpunk]

z.null() still doesn't seem to be generating proper values. This is related to #220 and #228.

OAS Document Version	API Version	API Output	Swagger UI	Correct & Works?
3.0	generateSchema(extendApi(z.null()), false, '3.0')	{ "type": "null" }		❌
3.1	generateSchema(extendApi(z.null()), false, '3.1')	{ "type": [ "string", "null" ], "enum": [ "null" ] }		❌
3.1	generateSchema(extendApi(z.null()), false, '3.0')	{ "type": "null" }		✅

When the overall document version is 3.0 and the version passed to generateSchema is 3.0, it generates a value that causes Swagger UI to print "Unknown type: null", which misleadingly looks like a string literal value.

When the overall document version is 3.1 and the version passed to generateSchema is 3.1, it generates a string literal value of "null", which is incorrect.

When the overall document version is 3.1 and the version passed to generateSchema is 3.0, Swagger UI works as expected... at least as far as handling z.null() is concerned. I don't know enough about the differences between 3.0 and 3.1 to trust this approach broadly.

[hexpunk]

It looks like when a fix was attempted in #235, the fix for version 3.0 vs 3.1 was applied backwards:

zod-plugins/packages/zod-openapi/src/lib/zod-openapi.ts

Lines 347 to 356 in 819fbec

	function parseNull({ zodRef, schemas, openApiVersion }: ParsingArgs<z.ZodNull>): SchemaObject {
	return merge(
	openApiVersion === '3.0' ? { type: 'null' as SchemaObjectType } : {
	type: ['string', 'null'] as SchemaObjectType[],
	enum: ['null'],
	},
	zodRef.description ? { description: zodRef.description } : {},
	...schemas
	);
	}

From the v3 spec:

null is not supported as a type

From the v3.1 spec:

Data types in the OAS are based on the types defined by the JSON Schema Validation Specification Draft 2020-12: "null" [...]

Switching the two cases should fix this issue.

However, I don't think { "type": [ "string", "null" ], "enum": [ "null" ] } is correct for 3.0 as "null" isn't a type there. I don't think this can be 100% solved for 3.0, but maybe { "type": "object", "nullable": true } would be closer to correct?

[hexpunk]

Further research suggests that for 3.0, the most correct type for null is { type: "string", "enum": [null] }. Swagger UI correctly shows this type as: