Correct handling when empty GraphQL requests arrive

[jmartisk]

Describe the bug

When an empty POST request arrives at the /graphql endpoint, the client receives a 500 Internal Server Error and this appears in the log:

2021-09-21 08:53:19,800 ERROR [io.qua.ver.htt.run.QuarkusErrorHandler] (vert.x-eventloop-thread-4) HTTP Request to /graphql/ failed, error id: 87f1643d-a786-485b-89c4-6d105c1c1ea8-1: java.lang.NullPointerException
	at java.base/java.io.StringReader.<init>(StringReader.java:50)
	at io.quarkus.smallrye.graphql.runtime.SmallRyeGraphQLExecutionHandler.getJsonObjectFromBody(SmallRyeGraphQLExecutionHandler.java:173)
	at io.quarkus.smallrye.graphql.runtime.SmallRyeGraphQLExecutionHandler.handlePost(SmallRyeGraphQLExecutionHandler.java:89)
	at io.quarkus.smallrye.graphql.runtime.SmallRyeGraphQLExecutionHandler.doHandle(SmallRyeGraphQLExecutionHandler.java:70)
	at io.quarkus.smallrye.graphql.runtime.SmallRyeGraphQLAbstractHandler.handleWithIdentity(SmallRyeGraphQLAbstractHandler.java:66)
	at io.quarkus.smallrye.graphql.runtime.SmallRyeGraphQLAbstractHandler.handle(SmallRyeGraphQLAbstractHandler.java:48)
	at io.quarkus.smallrye.graphql.runtime.SmallRyeGraphQLAbstractHandler.handle(SmallRyeGraphQLAbstractHandler.java:23)
	at io.vertx.ext.web.impl.BlockingHandlerDecorator.lambda$handle$0(BlockingHandlerDecorator.java:48)
	at io.vertx.core.impl.ContextImpl.lambda$null$0(ContextImpl.java:159)
	at io.vertx.core.impl.AbstractContext.dispatch(AbstractContext.java:100)
	at io.vertx.core.impl.ContextImpl.lambda$executeBlocking$1(ContextImpl.java:157)
	at org.jboss.threads.ContextHandler$1.runWith(ContextHandler.java:18)
	at org.jboss.threads.EnhancedQueueExecutor$Task.run(EnhancedQueueExecutor.java:2449)
	at org.jboss.threads.EnhancedQueueExecutor$ThreadBody.run(EnhancedQueueExecutor.java:1478)
	at org.jboss.threads.DelegatingRunnable.run(DelegatingRunnable.java:29)
	at org.jboss.threads.ThreadLocalResettingRunnable.run(ThreadLocalResettingRunnable.java:29)
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
	at java.base/java.lang.Thread.run(Thread.java:829)

an empty GET request yields 204 No Content

Expected behavior

I believe that the correct behavior for both cases is to issue 400 Bad Request, per the spec: https://github.com/graphql/graphql-over-http/blob/40a50a351a887799b3d1550b665b2101b26dcda4/spec/GraphQLOverHTTP.md#unparseable-or-invalid-request-body

Actual behavior

No response

How to Reproduce?

curl -v -H"Content-Type: application/json" -X POST localhost:8080/graphql
curl -v -H"Content-Type: application/json" -X GET localhost:8080/graphql/

Output of uname -a or ver

No response

Output of java -version

No response

GraalVM version (if different from Java)

No response

Quarkus version or git rev

No response

Build tool (ie. output of mvnw --version or gradlew --version)

No response

Additional information

No response

[quarkus-bot]

/cc @phillip-kruger