Having file with only PART header will cause wrong html structure in previous chapter

[cderv]

This comes from an investigation on an issue reported in tidymodels/TMwR#75

Step to reproduce

• Use the book demo
• Add a PART.Rmd file with this content

# (PART) Content {-}

• Use this _bookdown.yml

book_filename: "bookdown-demo"
language:
  ui:
    chapter_name: "Chapter "
delete_merged_file: true
rmd_files: [
  "index.Rmd",
  "01-intro.Rmd",
  "PART.Rmd",
  "02-literature.Rmd"
]

• Build the book

Issue

The chapter just before the PART header, the one call Introduction, will have an incorrect HTML structure leading to

• No navigation arrow on the side
• Wrong position and formatting of the reference and footnote section
  

The splitting process of bookdown does not handle well this case.
The section inside <div class 'page-inner'> will close too soon, causing the Reference part and the navigation arrows to be included at the end of the wrong div.

[jtbayly]

Perhaps related to #947

Sounds very similar, anyway.

[svanimpe]

I have the exact same issue. There's no forward navigation button on the chapter right before a PART header.

[svanimpe]

The workaround described here works for me: tidymodels/TMwR#75 (comment)

[cderv]

Duplicate of #662