fix: avoid rendering empty custom front matter

[rokuosan]

Summary

This PR fixes a rendering bug where an empty custom front matter object ({}) was emitted into the generated front matter block.

Root Cause

When custom front matter only contained fields already handled by the renderer, such as title, those known fields were consumed and removed before serialization. The remaining map became empty, but it was still marshaled and appended to the output, which produced {} in the front matter.

Changes

• Skip serializing additional front matter when no custom keys remain after applying known field overrides.
• Update renderer tests to assert that empty custom front matter is no longer emitted.

Impact

• Generated article front matter no longer includes a stray empty object.
• Cases with real custom front matter continue to render as before.

Validation

• go test ./pkg/core/...