BUG: Fix inconsistent error return values: return nil instead of []string{""} for GetTrustAnchorIDs methods

[Sukuna0007Abhi]

Issue Description:

Problem

Currently, several GetTrustAnchorIDs methods across different scheme handlers return []string{""} (slice with empty string) when encountering errors, instead of returning nil. This is inconsistent and can cause issues downstream where empty string IDs might be processed as valid identifiers.

Root Cause

The pattern of returning []string{""} on errors appears to be a leftover from earlier implementations and doesn't follow Go best practices for error handling. When an error occurs, the function should return nil for the slice and a proper error value.

Files Affected

The following files contain this pattern:

• trustedservices_grpc.go
• vts/schemes/cca-realm-token/store.go
• vts/schemes/cca-ssd-token/store.go
• vts/schemes/tpm-enacttrust/store.go
• vts/schemes/psa-iot/store.go
• vts/schemes/parsec-tpm/store.go
• vts/attestation_grpc.go
• Expected Behavior
• All GetTrustAnchorIDs methods should return nil (not []string{""}) when encountering errors, along with the appropriate error value.

Additional Context

This issue was identified during code review of PR #338. The mentor sir @setrofim noted: "refvalID should never be "". If they can occur now, then that is a bug that should be fixed elsewhere, not suppressed here."

This change will improve consistency across the codebase and prevent potential issues where empty string IDs might be treated as valid reference values

[Sukuna0007Abhi]

Address the PR, kallal, you created, I will make some changes also so I will be co-author there.

[kallal79]

Thanks for flagging, @Sukuna0007Abhi. Agreed—nil is cleaner and safer than []string{""}. I’ll update the PR accordingly. Let’s sync on edits; co-authoring sounds good.