create empty generator when no vulns present

Merge Request Description

Currently, images that are scanned by twistlock and present zero findings will not have a vulnerabilities key and will throw a key error when parsing the reports for VAT. To avoid this, when the vulnerabilities key is not present, we'll return an empty array to loop over which should create an empty generator. Converting an empty generator to a list results in an empty list, which is what we previously provided VAT when no twistlock findings were present.

Merge Request BOE

Risk

Low/Medium -- could be reverted but anything parsing the reports to VAT could introduce or remove potential findings

Rollback Plan

Revert MR

Testing

Locally tested