Markdown: Blank lines in the front YAML block are removed when merged

[Kuro Kurosaka]

After the fix of issue #725, blank lines in the 'front matter', the YAML block, are removed when merged if the parameters file with translateHeaderMetadata.b=true is used.

Before:

---

title: Back to the future

year: 1985
---

After:

---
title: Back to the future
year: 1985
---

[Clément Mouchet]

Addressed by #741