Dangerfile: Handle missing area in role

What does this MR do?

It's possible that area is not present in a role, then we should handle nil value too.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/66674

Screenshots

Does this MR meet the acceptance criteria?

Conformity

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• Documentation created/updated or follow-up review issue created
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Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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