Adjust test case to trigger a Jackson exception even with 2.18.3

Author: gsmet

integration-tests/funqy-amazon-lambda/src/test/java/io/quarkus/funqy/test/GreetTestBase.java

@@ -26,11 +26,13 @@ public void testGreet() {
     @Test
     public void testGreetNPE() {
         given()
+                .contentType("application/json")
+                .accept("application/json")
+                .body("[]")
                 .when()
                 .post()
                 .then()
                 .statusCode(500)
                 .body("errorMessage", containsString("Could not deserialize the provided message"));
     }
-
 }