Don't access CookieContainer unless needed (#16619)

Author: BrennanConroy

src/SignalR/clients/csharp/Client/test/UnitTests/HttpConnectionTests.cs

@@ -89,6 +89,14 @@ await WithConnectionAsync(
             Assert.Same(httpOptions.Credentials, httpClientHandler.Credentials);
         }
 
+        [Fact]
+        public void HttpOptionsCannotSetNullCookieContainer()
+        {
+            var httpOptions = new HttpConnectionOptions();
+            Assert.NotNull(httpOptions.Cookies);
+            Assert.Throws<ArgumentNullException>(() => httpOptions.Cookies = null);
+        }
+
         [Fact]
         public async Task HttpRequestAndErrorResponseLogged()
         {

src/SignalR/clients/csharp/Http.Connections.Client/src/HttpConnection.cs

@@ -517,13 +517,14 @@ private HttpClient CreateHttpClient()
                 {
                     httpClientHandler.Proxy = _httpConnectionOptions.Proxy;
                 }
-                if (_httpConnectionOptions.Cookies != null)
+
+                // Only access HttpClientHandler.ClientCertificates and HttpClientHandler.CookieContainer
+                // if the user has configured those options
+                // Some variants of Mono do not support client certs or cookies and will throw NotImplementedException
+                if (_httpConnectionOptions.Cookies.Count > 0)
                 {
                     httpClientHandler.CookieContainer = _httpConnectionOptions.Cookies;
                 }
-
-                // Only access HttpClientHandler.ClientCertificates if the user has configured client certs
-                // Mono does not support client certs and will throw NotImplementedException
                 // https://github.com/aspnet/SignalR/issues/2232
                 var clientCertificates = _httpConnectionOptions.ClientCertificates;
                 if (clientCertificates?.Count > 0)