Feature request: graphqlTypegen should generate undefined properties of Queries, not null

[corgrath]

With graphqlTypegen enabled, Gatsby generates Queries in gatsby-types.d.ts.

Here is an example of a generated query definition:

export const query = graphql`
  query TestPage {
  
    site {
      siteMetadata {
        baseURL
      }
    }
    
  }
`

type TestPageQuery = { readonly site: { readonly siteMetadata: { readonly baseURL: string | null } | null } | null };
}

In Gatsby's own example on their documentation page:

https://www.gatsbyjs.com/docs/how-to/local-development/graphql-typegen/

They give the example of use:

<p>Site title: {data.site?.siteMetadata?.title}</p>

However, if you were to pass title into a function while using strict TypeScript check, you need to check for both undefined and null.

This is because optional object?.property returns undefined why Gatsby's definition uses null.

Meaning, if you use strict type checking, your code will easily be bloated with code such as:

function prefixTitle( title: string | null | undefined, prefix: string ) {
   ...
}

And

if( title === null || title === null) {
throw new Error("Title is missing");
}

return <Header value={title}>

It would make developer's life a lot easier if the type definitions were undefined instead of null.