Fix z.coerce.date() rendering as nullable in OpenAPI

[coderabbitai]

The z.coerce.date() is being rendered as nullable in the OpenAPI schema.

Related issue: asteasolutions/zod-to-openapi#286

Location: src/schema.ts

// TODO: z.coerce.date() rendered as nullable
// See also: https://github.com/asteasolutions/zod-to-openapi/issues/286

This TODO should be addressed once the upstream issue is resolved.

Found in PR: #11
Comment: #11 (comment)