[BUG]: GraphQL mutation to remove relationship with null fails

[rderbier]

What version of Dgraph are you using?

22.0.1

What steps will reproduce the bug?

Follow the GraphQL Tour - Schema - page 7

Expected behavior and actual result.

mutation {
  updatePerson(input: {
    filter: { xid: { eq: "alice" } }
    remove: { manages: null }
  })
}

should remove the relationship manages. The request fails.
(it looks like it's used to work at a point before 21.03. : See the discuss threads.

Additional information

https://discuss.dgraph.io/t/removing-edges-using-null-does-not-work/15420
https://discuss.dgraph.io/t/mutation-provided-in-dgraph-graphql-tour-schema-doesnt-work/18137

Also related to #5711

[github-actions]

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