Ensure array type response for serialization of Vulnerability StateTransition, IssueLink and MergeRequestLinks

Purely to simplify the front end implementation, we should modify the serialisation of Vulnerabilities and Findings that include the StateTransition, IssueLink and MergeRequestLinks models to ensure their output is guaranteed as an array to reduce the complexity of null handling on the frontend.

Implementation Plan

• Modify the VulnerabilityEntity, Vulnerabilities::FindingEntity and Security::FindingEntity to enforce array type output of state_transitions, issue_links and merge_request_links