fix: allow required fields list to be specified as empty (#651)

[benweint]

Resolves #651

From that issue:

The OpenAPI spec section on the Schema object says:

The Schema Object allows the definition of input and output data types. These types can be objects, but also primitives and arrays. This object is a superset of the JSON Schema Specification Draft 2020-12.

For more information about the properties, see JSON Schema Core and JSON Schema Validation.

The JSON Schema Validation spec says of the required property:

The value of this keyword MUST be an array. Elements of this array, if any, MUST be strings, and MUST be unique.

An object instance is valid against this keyword if every item in the array is the name of a property in the instance.

Omitting this keyword has the same behavior as an empty array.

... which makes pretty clear that specifying an empty array for required is supported by JSON Schema, and thus by OpenAPI, and thus should be supported by this tool's model of OpenAPI.

[dbanty]

Thanks for both the detailed explanation and the PR!