fix: handle deserializing and writing empty security requirements #1426

[Paul Rijneveld @ Mendix (paulmendix)]

fix#1426.

make distinction between empty security requirements and no security requirements on an operation. empty security requirements are read as an empty list, no security requirements are read as null for OpenAPI v2/v3/v3.1. This is a breaking change, previously both cases were read as an empty list.

also includes a change to OpenApiOperation.SerializeInternal so it can serialize these two cases separately. this required a new method OpenApiWriterExtensions.WriteOptionalOrEmptyCollection.

includes unit tests, change to PublicApi.approved.txt to include the new method, and I removed a couple of unused usings and a typo in test name SerializeDocWithSecuritySchemeWithInlineReferencesWorks.

[Paul Rijneveld @ Mendix (paulmendix)]

I will contact the right people at my work so I can sign the CLA, this will take some time.

[Vincent Biret (baywet)]

Thanks for the contribution!
A couple of suggestions while we wait for the CLA.

[Paul Rijneveld @ Mendix (paulmendix)]

@microsoft-github-policy-service agree company="Mendix"

[Vincent Biret (baywet)]

Thanks for the contribution!

Maggie Kimani (@MaggieKimani1) for final review and merge

[Vincent Biret (baywet)]

Paul Rijneveld @ Mendix (@paulmendix) can you please handle the conflict before we review another time?
Maggie Kimani (@MaggieKimani1) when you merge, please ensure you squash, and that nowhere in the commit message we have !:, otherwise release please will try to major bump...

[Paul Rijneveld @ Mendix (paulmendix)]

Thanks. I addressed the conflict. Ready for final review Maggie Kimani (@MaggieKimani1)