Create custom schema definitions

[eojthebrave]

We should create custom schema definitions. This will help with issues where a site implements this theme, and for example, hasn't yet added a tutorial with an image (optional Drupal field). So the generated GraphQL schema won't contain the image field, and queries will break. With a custom schema we can at least have queries that return null.

This might help to generate them - https://www.gatsbyjs.org/packages/gatsby-plugin-schema-snapshot/?=schema