Remove GraphQL null field workaround

[khxia]

Before, when no events had the conferenceLink attribute, the GraphQL will not detect it when all of the events have that property as null. The workaround to this that we found was to set conferenceLink = ''; (look for it in the gatsby-node.js file). However, it seems that Gatsby has addressed this issue using a new schema customization API. So we need to use that instead.

This is the issue:
gatsbyjs/gatsby#6800

[khxia]

Instead of using createSchemaCustomization, we could try directly adding it into Gatsby sourceNodes.
This will still allow us to use the createTypes function.

https://www.gatsbyjs.com/docs/reference/config-files/gatsby-node/#createSchemaCustomization

https://www.gatsbyjs.com/docs/reference/graphql-data-layer/schema-customization/#creating-type-definitions

https://www.gatsbyjs.com/docs/reference/config-files/actions/#createTypes

This stuff could be wrong, but I came across it and wanted to make a note of this.

[khxia]

This issue is now also prevalent for link attribute in highlights.js