Fix isGraphQLQuery check in response-cache plugin

[fabsrc]

Fixes #2745

[intellix]

For the impatient using https://github.com/ds300/patch-package:

diff --git a/node_modules/apollo-server-plugin-response-cache/dist/ApolloServerPluginResponseCache.js b/node_modules/apollo-server-plugin-response-cache/dist/ApolloServerPluginResponseCache.js
index 67a298a..7130a03 100644
--- a/node_modules/apollo-server-plugin-response-cache/dist/ApolloServerPluginResponseCache.js
+++ b/node_modules/apollo-server-plugin-response-cache/dist/ApolloServerPluginResponseCache.js
@@ -26,7 +26,9 @@ function cacheKeyString(key) {
     return sha(JSON.stringify(key));
 }
 function isGraphQLQuery(requestContext) {
-    return requestContext.operation.operation === 'query';
+  return (
+    requestContext.operation && requestContext.operation.operation === 'query'
+  );
 }
 function plugin(options = Object.create(null)) {
     return {

[fabsrc]

@abernix or @glasser: could you please have a look at this? 🙂

[abernix]

Thank you so much!

(And, if I'm not mistaken, pleasure to run into you on the streets of Berlin and discuss this PR! 😺)

[glasser]

Good catch, thanks!