Check for missing errors in GraphQlTester#executeAndVerify

Prior to this commit, the `GraphQlTester#executeAndVerify` method would
check that an `"$.errors"` JSON path exists within the response and
contains no value.

This should fail with valid GraphQL responses, as the spec says that a
response with no error should not contain the errors map at all:
http://spec.graphql.org/draft/#sec-Errors

This commit changes the assertion and instead checks for the absence of
the errors map in the response.

Fixes gh-318

spring-graphql-test/src/main/java/org/springframework/graphql/test/tester/DefaultGraphQlTester.java

@@ -170,7 +170,7 @@ final class DefaultGraphQlTester implements GraphQlTester {
 
 		@Override
 		public void executeAndVerify() {
-			execute().path("$.errors").valueIsEmpty();
+			execute().path("$.errors").pathDoesNotExist();
 		}
 
 		@Override

spring-graphql-test/src/test/java/org/springframework/graphql/test/tester/GraphQlTesterTests.java

@@ -186,6 +186,17 @@ public class GraphQlTesterTests extends GraphQlTesterTestSupport {
 		assertThat(input.getVariables()).containsEntry("keyOnly", null);
 	}
 
+	@Test
+	void errorsEmptyOnExecuteAndVerify() {
+
+		String document = "{me {name, friends}}";
+		setMockResponse("{\"me\": {\"name\":\"Luke Skywalker\", \"friends\":[]}}");
+
+		graphQlTester().document(document).executeAndVerify();
+
+		assertThat(requestInput().getDocument()).contains(document);
+	}
+
 	@Test
 	void errorsCheckedOnExecuteAndVerify() {