Better null check in getSchemaFromResponse

[wing328]

Better null check in getSchemaFromResponse

A follow-up PR to #16986

FYI @OpenAPITools/generator-core-team

PR checklist

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• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./bin/utils/export_docs_generators.sh
  

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  Commit all changed files.
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