Guard comparison against null path, parameter, and schema collections

[Jerry-Johnson-3]

Summary

• Treat missing path parameters, operation maps, and component schemas as empty dictionaries/lists so document comparison does not throw.
• Coalesce null operation parameters lists and resolve $ref parameters with Components?.Parameters.
• Add focused unit tests that mutate parsed documents to the null collection states OpenAPI.NET can leave in memory.

Spec

OpenAPI 3.0 treats unspecified fields as optional. A path item may have no operations or parameters; components and its maps may be omitted. $ref resolution must tolerate a missing components.parameters map.

• OAS 3.0.3 Schema — fields that are not REQUIRED are OPTIONAL
• Path Item Object — “A Path Item MAY be empty”; get/post/… and parameters are not required
• Operation Object — parameters is not required (responses is)
• Components Object — schemas / parameters maps are not required; components itself is optional on the OpenAPI Object
• Reference Object / JSON Reference — $ref targets components.parameters when present

Testing

• Red: the five new tests failed with NullReferenceException / ArgumentNullException in CompareParameters, CompareOperations, CompareSchemas, and ParameterComparator.Compare.
• Green: the same tests passed after the null coalescing, and the full 0.8.3 suite stayed green (101 tests).