Slugifying non-ascii headers make duplicate permalinks

[Joe-noh]

Problem

I have following OpenAPI yaml which markdown headers are composed of non-ascii characters.

swagger: "2.0"
info:
  title: サンプルAPI
  description: |
    # サンプルAPI

    サンプルのAPIドキュメントです。

    ## 利用手順

    はじめにデベロッパー登録してください。

    ## エラー

    以下のような形式でエラーを返します。

ReDoc will slugify these headers to make permanent links. However non-ascii-only header will be converted into same section names like this. They don't work well as permanent links.

# サンプルAPI -> slugify -> "API" ->"/#section/API"

## 利用手順 -> slugify -> "" -> "/#section/API/"

## エラー -> slugify -> "" -> "/#section/API/"

I committed Joe-noh@32f2f7f. This uses original header title if slugified title is empty. I guess this is simplest solution for this problem. Please let me know if there are any other good options.

Thanks. Joe.

[RomanHotsiy]

@Joe-noh thanks for the PR
merged

Will notify you once new version is published

[Joe-noh]

Thank you!