Unable to lint open api if it is wrapped by object

[school-coder]

Describe the bug
When openapi is wrapped in a object ( JSON or YAML), extends: [spectral:oas] is not able to be reused.

To Reproduce

1. Given this document,
   
   spec attribute needs to comply with spectral:oas ruleset.

2. When i try to start the node js server, with the below rule,
   

I am getting the following error

RulesetValidationError: the rule must have at least "given" and "then" properties

Expected behavior
When a rule extends a certain rule set (as we have done for 'spectral:oas' for 'spec' attribute) for a given attribute, linting needs to be done for that attribute and corresponding linting responses needs to be provided. 'then' must be optional if 'extends' is provided.