[cubahno]

A required property declared as anyOf: [T, {type: "null"}], which Pydantic and FastAPI emit for T | None without a default, was generated as a plain T. Building the union saw the null option, but the object's property loop then recomputed nullability from the property's own type, which an anyOf doesn't have. So a null from the server decoded to the zero value and was sent back as 0001-01-01T00:00:00Z or "". Separately, every required nullable property got omitempty, so a nil value dropped a key the schema requires instead of writing null. That one already applied to type: [T, "null"] and nullable: true.

What changes

• A property whose anyOf or oneOf lists {type: "null"} next to a single other type is nullable, so a required one becomes a pointer: UpdatedAt *time.Time.
• A required property gets no omitempty, so a nil pointer encodes as null. That holds for type: [T, "null"] and nullable: true too, and for the MarshalJSON generated for types with additionalProperties or a union next to their properties.
• readOnly and writeOnly properties still count as optional and keep omitempty. Optional properties, unions of two or more types besides null, and request parameters generate as before.
• For custom templates: Constraints.RequiredKey says whether the object requires the property's key, and Property.OmitsNil whether a nil value leaves the key out.

Fixes #162