Avoid DOCX section break with initial heading using --top-level-division

I think it would be ideal not to generate a section break using `--top-level-division` for the first heading if it is the initial item in the document, should it be feasible to detect this.

For example, this results in a document with a blank first page:

```markdown
pandoc -o chapter.docx --top-level-division=chapter << EOT

# Chapter 1

Sit tenetur aspernatur numquam et aliquam.

EOT
```