index.html fails to be created silently when there is no prefix chapter

[ohemelaar]

When building a book that has no prefix chapter before other titles are defined in SUMMARY.md the index.html file isn't built.

Steps to reproduce:

cd /tmp
mdbook init --ignore git --title 'My Book' my-book
cd my-book
sed -i '3i# Part 1' src/SUMMARY.md
cat src/SUMMARY.md 
# # Summary
# 
# # Part 1
# - [Chapter 1](./chapter_1.md)
mdbook build
ls book/index.html
# ls: cannot access 'book/index.html': No such file or directory

The docs say:

Before the main numbered chapters, prefix chapters can be added...

My understanding is that prefix chapters are thus optional, but mdbook doesn't render correctly without them.

[ISSOtm]

According to the code, the first item in the book is supposed to be rendered a second time as index.html which seems to be undocumented.

My guess as to the cause for this bug is that the first item, if there are no prefix chapters, is the section title. So this should trigger with e.g. a separator as well.