Fix ambiguity around when schema definition may be omitted

[benjie]

This PR implements the tests and fixes necessary for Spec RFC #987.

The main changes

This PR is made of three main commits:

1. Test printing a schema that has Query, Mutation and Virus types, but only supports query operations (via the Query type) and does not support mutation operations.
2. Test parsing this same schema text, and assert that the schema does not have a mutation type.
3. Fix the printing of the schema.

Unnecessary test?

The fourth commit addresses the following failing test that I didn't know what to do about:

graphql-js/src/utilities/__tests__/buildClientSchema-test.ts

Lines 65 to 81 in 76e47fc

	it('builds a schema without the query type', () => {
	const sdl = dedent`
	type Query {
	foo: String
	}
	`;
	const schema = buildSchema(sdl);
	const introspection = introspectionFromSchema(schema);
	// @ts-expect-error
	delete introspection.__schema.queryType;
	const clientSchema = buildClientSchema(introspection);
	expect(clientSchema.getQueryType()).to.equal(null);
	expect(printSchema(clientSchema)).to.equal(sdl);
	});

I don't understand the reasoning behind this test. It seems to me that the schema output from this should instead be:

schema {} type Query { foo: String }

since the schema has no operation types (but uses the default names); however this is unparseable (because {} is unparseable). Perhaps @IvanGoncharov or @leebyron can shed some light on the reasoning behind this test? I've worked around it by special casing it, but I'd prefer to remove the special case code and also remove the test case.

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[benjie]

cc @graphql/graphql-js-reviewers

[benjie]

Going to close/reopen to see if Netlify retries 🤷‍♂️

[spawnia]

Works nicely when ported to PHP: webonyx/graphql-php#1303.