Correctly handle t.undefined in openapi-generator

[bitgopatmcl]

Undefined values are represented in the IR and should get optimized down to optional or missing properties. What was overlooked is using t.undefined directly rather than going through h.optional(...). This should address that.

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🎉 This PR is included in version @api-ts/openapi-generator@2.7.1 🎉

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🎉 This PR is included in version @api-ts/typed-express-router@1.1.1 🎉

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🎉 This PR is included in version @api-ts/express-wrapper@1.0.21 🎉

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🎉 This PR is included in version @api-ts/superagent-wrapper@1.1.12 🎉

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🎉 This PR is included in version @api-ts/io-ts-http@2.4.1-beta.1 🎉

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