Lib_openapi: fix handling of `null` option within Combine constructor

Context

This commit fixes a bug where Null elements in Json schemas would not be processed correctly when being translated to OpenAPI schemas.

The version of OpenAPI we use does not handle null elements in disjunctions, so instead we use a nullable property on said disjunctions, and don't convert null elements from Json schemas to OpenAPI.

The problem was that null elements in disjunctions (i.e. the Combine element kind) can be represented either by an element with the Null kind, or by the following object:

{ /* None */
  "none": null }

This commit adds the correct processing of the latter representation to the convert_element function.

Part of #3281 (closed)

Manually testing the MR

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