fix: Add minLength to `identifiers` required fields

What does this MR do?

As a follow-up to !56 (merged) we should make all required fields within identifiers expect a minLength of 1. It's unclear why we didn't do this previously and whether it was missed or purposeful.

This bug was discovered in investigating gitlab-org/gitlab#323221 (closed) in which an empty identifier[].value within a SAST report is halting StoreSecurityReportsWorker, leading to a failure to parse subsequent security reports

Availability and Testing

• Review and add/update tests for this feature/bug

Approvals

• groupcomposition analysis @adamcohen
• groupstatic analysis @dsearles
• groupdynamic analysis @craigmsmith
• ~"group::fuzz testing" @sethgitlab