Allow null arguments from HubConnection (#11241)

src/SignalR/clients/csharp/Client.Core/src/HubConnection.cs

@@ -627,7 +627,7 @@ namespace Microsoft.AspNetCore.SignalR.Client
 
             for (var i = 0; i < args.Length; i++)
             {
-                if (ReflectionHelper.IsStreamingType(args[i].GetType()))
+                if (args[i] != null && ReflectionHelper.IsStreamingType(args[i].GetType()))
                 {
                     if (readers == null)
                     {

src/SignalR/clients/csharp/Client/test/FunctionalTests/HubConnectionTests.cs

@@ -138,6 +138,34 @@ namespace Microsoft.AspNetCore.SignalR.Client.FunctionalTests
             }
         }
 
+        [Theory]
+        [MemberData(nameof(HubProtocolsList))]
+        public async Task CanSendNull(string protocolName)
+        {
+            var protocol = HubProtocols[protocolName];
+            using (StartServer<Startup>(out var server))
+            {
+                var connection = CreateHubConnection(server.Url, "/default", HttpTransportType.LongPolling, protocol, LoggerFactory);
+                try
+                {
+                    await connection.StartAsync().OrTimeout();
+
+                    var result = await connection.InvokeAsync<string>(nameof(TestHub.Echo), null).OrTimeout();
+
+                    Assert.Null(result);
+                }
+                catch (Exception ex)
+                {
+                    LoggerFactory.CreateLogger<HubConnectionTests>().LogError(ex, "{ExceptionType} from test", ex.GetType().FullName);
+                    throw;
+                }
+                finally
+                {
+                    await connection.DisposeAsync().OrTimeout();
+                }
+            }
+        }
+
         [Theory]
         [MemberData(nameof(HubProtocolsAndTransportsAndHubPaths))]
         [LogLevel(LogLevel.Trace)]