fix(api): emit empty list fields in REST JSON responses

[sudhishmk]

Fixes #1076

Summary

• Enable EmitUnpopulated on the gRPC gateway JSON marshaler so empty list responses include records/results and next_page_token fields.

Changes

The gRPC gateway JSON marshaler omitted zero-value fields, so empty ListRecords/ListResults responses serialized as {} instead of {"records":[],"next_page_token":""}. Enable EmitUnpopulated to match the OpenAPI spec and grpc-gateway defaults.

• Add unit tests for empty ListRecordsResponse and ListResultsResponse marshaling.
  Fixes List Records returns {} #1076

Test plan

• go test ./cmd/api/ -run 'TestAPIJSONMarshaler' -v
• make fmt
• make golangci-lint
• ./test/presubmit-tests.sh --unit-tests
• [ ]
  /kind bug

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[sudhishmk]

/release-note-none