Error show and invalid navigation if a summary item has no reference

[le.chang]

Summary

Sometimes, a summary item without a reference is only a container of sub items, this case result in error showing and invalid navigation.

• HonKit version: 4.0.4

Step to reproduce

1. create a SUMMARY.md like below, make sure references are valid.
2. run honkit serve

• Link to code example:

# Summary

* [Introduction](README.md)

## Volume I

* Chapter One

  * [section1](chapter1/section1.md)

  * [section2](chapter1/section2.md)

* Chapter Two

  * [section1](chapter2/section1.md)

  * [section2](chapter2/section2.md)

## Volume II

Expected results

Show 'Chapter One' and 'Chapter Two'

Actual results

'Chapter One' show 'section1', 'Chapter Two' show 'section2', and navigation is invalid when go to 'Chapter One' page.