Add tests for empty response bodies

[kalashnikovisme]

Summary

• add regression specs that ensure an empty response body triggers the existing validation error when a body schema is declared
• document in the README how to handle empty responses so users avoid defining a body for them

Testing

• bundle exec rspec (fails: bundler: command not found: rspec; bundle install then failed with Gem::Net::HTTPClientException 403 "Forbidden")

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Codex Task