gatsby: gatsby-remark-images error if using markdownCaptions: true

Description

There seems to have been a regression and an error similar to the one reported in #16703 and fixed in #19288 is effected if markdownCaptions: true is specified in the gatsby-remark-images configuration within gatsby-config.js

The error is the same, but the reported call stack is a little different.

Steps to reproduce

1. Create a new Gatsby site using gatsby-starter-blog:

    gatsby new markdown-caption-repro https://github.com/gatsbyjs/gatsby-starter-blog
   

2. Run yarn upgrade

3. Specify markdownCaptions in the gatsby-remark-images block

    resolve: `gatsby-remark-images`,
    options: {
      markdownCaptions: true,
      maxWidth: 590,
      showCaptions: true,
    },
   

4. Run yarn develop

Expected result

The page content should be generated and the development site should be available.

Actual result

 ERROR

UNHANDLED REJECTION Cannot read property 'contentDigest' of undefined



  TypeError: Cannot read property 'contentDigest' of undefined

  - extend-node-type.js:59 htmlAstCacheKey
    [narkdown-caption-repro]/[gatsby-transformer-remark]/extend-node-type.js:59:87

  - extend-node-type.js:318 getHTMLAst
    [narkdown-caption-repro]/[gatsby-transformer-remark]/extend-node-type.js:318:41

  - extend-node-type.js:343 Object.getHTML [as generateHTML]
    [narkdown-caption-repro]/[gatsby-transformer-remark]/extend-node-type.js:343:27

  - index.js:150 getImageCaption
    [narkdown-caption-repro]/[gatsby-remark-images]/index.js:150:21

  - index.js:301 generateImagesAndUpdateNode$
    [narkdown-caption-repro]/[gatsby-remark-images]/index.js:301:52


not finished run queries - 1.251s
not finished Generating image thumbnails - 1.383s

Environment

  System:
    OS: Windows 10 10.0.17763
    CPU: (4) x64 Intel(R) Celeron(R) N4100 CPU @ 1.10GHz
  Binaries:
    Node: 12.11.0 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.21.0 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 6.11.3 - C:\Program Files\nodejs\npm.CMD
  Languages:
    Python: 2.7.16 - /c/Programs/Python27/python
  Browsers:
    Edge: 44.17763.831.0
  npmPackages:
    gatsby: ^2.18.4 => 2.18.7 
    gatsby-image: ^2.2.34 => 2.2.34 
    gatsby-plugin-feed: ^2.3.22 => 2.3.23 
    gatsby-plugin-google-analytics: ^2.1.29 => 2.1.29 
    gatsby-plugin-manifest: ^2.2.30 => 2.2.31 
    gatsby-plugin-offline: ^3.0.24 => 3.0.27 
    gatsby-plugin-react-helmet: ^3.1.16 => 3.1.16 
    gatsby-plugin-sharp: ^2.3.4 => 2.3.5 
    gatsby-plugin-typography: ^2.3.18 => 2.3.18 
    gatsby-remark-copy-linked-files: ^2.1.31 => 2.1.31 
    gatsby-remark-images: ^3.1.34 => 3.1.35 
    gatsby-remark-prismjs: ^3.3.25 => 3.3.25 
    gatsby-remark-responsive-iframe: ^2.2.28 => 2.2.28 
    gatsby-remark-smartypants: ^2.1.17 => 2.1.17 
    gatsby-source-filesystem: ^2.1.39 => 2.1.40 
    gatsby-transformer-remark: ^2.6.38 => 2.6.39 
    gatsby-transformer-sharp: ^2.3.6 => 2.3.7