Implement Type Definitions to handle "optional" fields

[jamesvidler]

You want to optionally use your field "postAuthor" and right now it is not used anywhere. Therefore Gatsby can't infer the type and add it to the GraphQL schema. A quick fix is to add a least one entry with that field ("dummy content")

It is recommended to explicitly type your GraphQL schema if you want to use optional fields. This way you don't have to add the mentioned "dummy content". Visit our docs to learn how you can define the schema for "agilityPostAgilityFields":
https://www.gatsbyjs.org/docs/schema-customization/#creating-type-definitions