Fix v4.2.0 regression in variables: null (et al)

In v4.2.0 (#7171) we changed POST handling to be stricter if
`operationName`, `variables`, or `extensions` were provided with a
surprising data type. This was intended to pass more of the optional
recommendations of the GraphQL Over HTTP spec as tested by the
graphql-http audit suite. However, we were overzealous and also banned
providing these parameters as an explicit `null`, which is documented by
the spec as legitimate. (And some clients, such as FIXME, actually send
`variables: null` in practice.)

We added explicit tests for this to the `graphql-http` test suite
(graphql/graphql-http#28) and this commit allows
these `null`s again.

Fixes #7200.

Author: glasser

.changeset/red-cats-clap.md

@@ -0,0 +1,6 @@
+---
+'@apollo/server-integration-testsuite': patch
+'@apollo/server': patch
+---
+
+Fix v4.2.0 (#7171) regression where `"operationName": null`, `"variables": null`, and `"extensions": null` in POST bodies were improperly rejected.

package-lock.json

@@ -36,7 +36,7 @@
     "@josephg/resolvable": "^1.0.1",
     "body-parser": "^1.20.0",
     "express": "^4.18.1",
-    "graphql-http": "1.8.0",
+    "graphql-http": "1.9.0",
     "graphql-tag": "^2.12.6",
     "loglevel": "^1.8.0",
     "node-fetch": "^2.6.7",

packages/integration-testsuite/package.json

@@ -155,6 +155,7 @@ export async function runHttpQuery<TContext extends BaseContext>({
 
       if (
         'extensions' in httpRequest.body &&
+        httpRequest.body.extensions !== null &&
         !isStringRecord(httpRequest.body.extensions)
       ) {
         throw new BadRequestError(
@@ -164,6 +165,7 @@ export async function runHttpQuery<TContext extends BaseContext>({
 
       if (
         'variables' in httpRequest.body &&
+        httpRequest.body.variables !== null &&
         !isStringRecord(httpRequest.body.variables)
       ) {
         throw new BadRequestError(
@@ -173,6 +175,7 @@ export async function runHttpQuery<TContext extends BaseContext>({
 
       if (
         'operationName' in httpRequest.body &&
+        httpRequest.body.operationName !== null &&
         typeof httpRequest.body.operationName !== 'string'
       ) {
         throw new BadRequestError(