Fixed new warnings

Author: Bart Koelman

test/JsonApiDotNetCoreExampleTests/IntegrationTests/ResourceHooks/ResourceHookTests.cs

@@ -77,7 +77,8 @@ public async Task Can_create_user_with_password()
             User responseUser = GetResponseDeserializer().DeserializeSingle<User>(responseDocument).Data;
             var document = JsonConvert.DeserializeObject<Document>(responseDocument);
 
-            document.SingleData.Attributes.Should().NotContainKey("password");
+            document.Should().NotBeNull();
+            document!.SingleData.Attributes.Should().NotContainKey("password");
             document.SingleData.Attributes["userName"].Should().Be(newUser.UserName);
 
             await _testContext.RunOnDatabaseAsync(async dbContext =>

test/JsonApiDotNetCoreExampleTests/IntegrationTests/Serialization/SerializationTests.cs

@@ -283,7 +283,9 @@ public async Task Cannot_get_unknown_primary_resource_by_ID()
             httpResponse.Should().HaveStatusCode(HttpStatusCode.NotFound);
 
             var jObject = JsonConvert.DeserializeObject<JObject>(responseDocument);
-            string errorId = jObject["errors"].Should().NotBeNull().And.Subject.Select(element => (string)element["id"]).Single();
+            jObject.Should().NotBeNull();
+
+            string errorId = jObject!["errors"].Should().NotBeNull().And.Subject.Select(element => (string)element["id"]).Single();
 
             responseDocument.Should().BeJson(@"{
   ""errors"": [

test/TestBuildingBlocks/HttpResponseMessageExtensions.cs

@@ -47,7 +47,7 @@ private static async Task<string> GetFormattedContentAsync(HttpResponseMessage r
                 {
                     if (text.Length > 0)
                     {
-                        return JsonConvert.DeserializeObject<JObject>(text).ToString();
+                        return JsonConvert.DeserializeObject<JObject>(text)?.ToString();
                     }
                 }
 #pragma warning disable AV1210 // Catch a specific exception instead of Exception, SystemException or ApplicationException