#915 Allow missing security schemes

Fixes #737

A missing components.securitySchemes is valid OpenAPI, and as such should be accepted for diffing, rather than throwing an exception as it's currently doing.

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Summary by cubic

Allow diffing OpenAPI documents that omit components.securitySchemes, treating them as valid and unchanged instead of throwing errors. Fixes #737.

• Bug Fixes
  • Added null-safe handling for missing components.securitySchemes in security diff logic; treat absence as a no-op.
  • Updated tests to assert unchanged for specs without security schemes; still throw when a security requirement references a non-existent scheme.