Support type: 'null' as OpenAPIValueContainer

[brandonbloom]

Motivation

OpenAPIKit's handling of type: 'null' was problematic with older Yams versions. See https://github.com/mattpolzin/OpenAPIKit/pull/366/files for a test demonstrating that & dependency version bump.

Similarly, swift-openapi-generator explicitly does not "support" type: 'null'. As part of searching for something to address #513 I have added such support for explicit nulls. This gives one more sort of input schema where swift-openapi-generator produces some useful output instead of skipping a schema.

Modifications

• Bumped minimum supported Yams version.
• Returntrue from isSchemaSupported for type: 'null' schemas.
• Defined .null to have a builtint ype of OpenAPIValueContainer

Result

Nothing should change for existing users because type: null schemas were previously unsupported. Going forward, usages of these schemas is supported.

Test Plan

New test included.

[czechboy0]

Hmm I'm still unsure whether we want to promote null to a JSON value here. Let's discuss in the other PR first, then we can come back to this.

[brandonbloom]

I also considered the singleton instance of NSNull, or some new enum type with one case, or () or something. It wasn't obvious if there was a more appropriate type for this in Swift.

[czechboy0]

Hmm maybe we should introduce an typealias OpenAPINull = Void in the runtime library, and use that? What do you think? cc @simonjbeaumont

OpenAPIValueContainer isn't appropriate, I think, as it erases our knowledge of it being null, forcing the adopter to check again later. We should be able to avoid that.

[czechboy0]

Based on the discussion in the other thread, I think we should not merge this PR, and instead we should simply filter out null from anyOf/allOf schemas, as OpenAPIKit will already represent the nullability on the parent schema.