fix(golang): make object values nullable when specified

[rubenhoenle]

relates to #24264

Detailed description and testing instructions can be found in the GitHub issue.

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---

Summary by cubic

Fixes Go codegen to respect nullable for additionalProperties values. Generates map[string]*T when the value schema is nullable: true, skips pointers for map/slice values, and defaults to interface{} when no value schema is provided.

• Bug Fixes
  • Map values use pointers only when the inner schema is nullable and not a map/slice.
  • Nested objects honor nullable on inner values (e.g., map[string]map[string]*string).

^{Written for commit e3624b6. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}