fix(GraphQL): fix interface conversion panic in v20.03

[abhimanyusinghgaur]

Fixes DGRAPH-1787.

This PR fixes an issue, where if some user accidentally deleted the dgraph.type edge for the GraphQL schema node and simultaneously added another GraphQL schema node with dgraph.type edge and restarted the alpha, it would end up crashing with a panic saying panic: interface conversion: interface {} is nil, not string.

Steps to reproduce:

• Start zero and alpha.
• Run following mutation:

  {
    set {
      _:newSchema <dgraph.type> "dgraph.graphql" .
    }
    delete {
      <0x1> <dgraph.type> * .
    }
  }
  

  Note that <0x1> is the uid of existing GraphQL schema node. What we did in this mutation is to remove the dgraph.type edge from the existing schema node, and add a new node with just dgraph.type edge.
• Restart alpha.
• It will panic, and crash.
  The reason it happens is that the code in 20.03 used to do the initial GraphQL schema upsert using the type query, and when you restart the alpha, it will find the new node which doesn't have the dgraph.graphql.schema edge. So, it comes as nil in query, and hence the panic.

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[martinmr]

Include steps to reproduce in PR description.

Reviewed 1 of 1 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @abhimanyusinghgaur and @manishrjain)

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graphql/admin/admin.go, line 453 at r1 (raw file):

existingGQLSchemaQryResp

this can be gqlSchemaQryResp