Fix error message when no query is defined

[Shane32]

Currently a ARGUMENT_NULL error is returned to the client when a query request arrives for a schema with no root query type. This PR aligns the error message with the similar case for mutation and subscription types, until such time as we decide to disallow schemas without a root query type (in accordance with the spec).

Note that there is an upcoming RFC to the spec for a validation rule to address this condition:
graphql/graphql-spec#955

[Shane32]

Problem: this means that without a query type, introspection queries cannot be performed anymore with this PR.

And we have a test that validates an introspection query for a schema without a query type !!!

[Shane32]

We should consider revising our schema introspection to require a query root type, as the spec requires -- and changing all of the tests as required.

[sungam3r]

We should consider revising our schema introspection to require a query root type, as the spec requires

I remember how I read discussions regarding it ). Hot topic.

[codecov-commenter]

Codecov Report

Merging #3170 (a35632c) into master (ec7cef0) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3170      +/-   ##
==========================================
- Coverage   84.43%   84.43%   -0.01%     
==========================================
  Files         371      371              
  Lines       16087    16099      +12     
  Branches     2599     2604       +5     
==========================================
+ Hits        13583    13593      +10     
  Misses       1882     1882              
- Partials      622      624       +2     

Impacted Files	Coverage Δ
src/GraphQL/Execution/ExecutionStrategy.cs	85.19% <100.00%> (+0.59%)	⬆️
src/GraphQL/GraphQLBuilderExtensions.cs	95.86% <0.00%> (-0.80%)	⬇️
src/GraphQL/Execution/Nodes/ExecutionNode.cs	83.51% <0.00%> (ø)

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[Shane32]

@sungam3r I fixed the tests. I see you already approved this PR, but I'll wait to merge it until tomorrow, so you have a chance to see my additional changes if desired.

[sungam3r]

I reviewed again. 👍

This dummy field for query looks pointless though. I'm still hope that spec wg will find better solution.