fix(gatsby-source-drupal): sanity check before accessing field

[mglaman]

Description

I do not know why, but https://d2.api.demo.centarro.io/jsonapi/action/action returned the following error when trying to build a project w/ gatsby-source-drupal

 ERROR #11321  PLUGIN
"gatsby-source-drupal" threw an error while running the sourceNodes lifecycle:
Cannot read property 'next' of undefined
  TypeError: Cannot read property 'next' of undefined
  - gatsby-node.js:109 getNext
    [gatsbyjs_commerce_demo]/[gatsby-source-drupal]/gatsby-node.js:109:24

when unauthenticated. There is a links object. But it is acting like it is null

[pieh]

I'm not sure about this specific problem, but addtional sanity check always make sense. Thanks @mglaman!

[mglaman]

@pieh rad, thanks. Technically it's there when I view it manually 🤷‍♂️I'll be going through the plugin a lot in the next few weeks and try to run the debugger along with it.

[mglaman]

Okay, so this is actually due to to the JSON:API Schema (https://www.drupal.org/project/jsonapi_schema) module. On /jsonapi the describedby route is taken into consideration

    "describedby": {
      "href": "https://d2.api.demo.centarro.io/jsonapi/schema"
    },

This route doesn't follow the jsonapi spec. I'll open an issue for this specifically