Return an empty compact envelope for zero-match callees queries

[Widthdom]

Summary

callees can fabricate one empty projected row for an exact zero-match query in compact JSON mode. The equivalent references and callers queries return the expected empty compact envelope.

Reproduction

cdidx callees DefinitelyNoSuchSymbol --exact-name --json --compact \
  --fields path,line --limit 1 --max-json-bytes 8192 --db .cdidx/codeindex.db

The same behavior is observable without --fields.

Actual

The response uses the bounded envelope and reports one result, for example:

{"metadata":{"command":"callees","result_count":1,"total_count":1,"format":"compact"},"results":[{}]}

Expected

The response should preserve the compact location envelope and report no results:

{"format":"compact","count":0,"results":[]}

Notes

This was observed while reviewing #5094, but is pre-existing and independent of projected --body validation, so it is intentionally left out of that fix.

[Widthdom]

Work started on branch fix-issue5128.