Empty collections for default values are omitted during serialization

[baywet]

Describe the bug
Using v2.X, whenever a default value is an empty collection/array, it's ignored during serialization.
While this kind of behaviour might be ok for structural properties/collections defined in the specification (since it might be semantically equivalent), it's not for the default values.

OpenApi File To Reproduce

openapi: 3.1.0
info:
 title: (title)
 version: 0.0.0
paths: {}
components:
 schemas:
 Bar:
 type: object
 required:
 - foo
 properties:
 foo:
 type: array
 items:
 type: string
 default: []

(parsing and serializing back results in the default value disappearing)

Expected behavior
The default empty collection to be present in the serialized representation.

I intend to follow up with a pull request for this issue.

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