fix(worma): guard getParserSchemaType against undefined schema

[SAY-5]

Closes #830.

An array schema with no items makes arrayTypeParser call ctx.next(undefined, ...), so getParserSchemaType receives undefined and throws Cannot read properties of undefined (reading 'type') at parsers/utils.ts when it reaches schema.type.

Return 'null' early when the schema is nullish, matching the existing fallback for typeless schemas, so alova gen no longer crashes on such documents. Added a unit test in test/parsers/utils.spec.ts covering the undefined case.

[changeset-bot]

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