Fix async/sync behavioral inconsistency for top-level !ignore documents

[Copilot]

loadAndResolveSync returned undefined for a top-level !ignore document while loadAndResolve returned null, breaking public API consistency between the two variants.

Changes

• src/resolver.ts: Added early-return null guard in loadAndResolveSync for undefined/null parsed roots, matching the existing check in loadAndResolve
• test/ignore.test.ts: Added test covering loadYamlWithReferencesSync returning null for a top-level !ignore document

// Before: inconsistent results for the same input
await loadYamlWithReferences("ignored.yaml");  // null  ✓
      loadYamlWithReferencesSync("ignored.yaml");  // undefined  ✗

// After: consistent
await loadYamlWithReferences("ignored.yaml");  // null  ✓
      loadYamlWithReferencesSync("ignored.yaml");  // null  ✓

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