Fix NullReferenceException for dynamic repositories without authentication

Author: GeertvanHorrik

src/GitTools.Core/GitTools.Core.Shared/Git/DynamicRepositories.cs

@@ -170,15 +170,19 @@ static void CheckoutCommit(IRepository repo, string targetCommit)
         static void CloneRepository(string repositoryUrl, string gitDirectory, AuthenticationInfo authentication)
         {
             Credentials credentials = null;
-            if (!string.IsNullOrWhiteSpace(authentication.Username) && !string.IsNullOrWhiteSpace(authentication.Password))
-            {
-                Log.Info(string.Format("Setting up credentials using name '{0}'", authentication.Username));
 
-                credentials = new UsernamePasswordCredentials
+            if (authentication != null)
+            {
+                if (!string.IsNullOrWhiteSpace(authentication.Username) && !string.IsNullOrWhiteSpace(authentication.Password))
                 {
-                    Username = authentication.Username,
-                    Password = authentication.Password
-                };
+                    Log.Info(string.Format("Setting up credentials using name '{0}'", authentication.Username));
+
+                    credentials = new UsernamePasswordCredentials
+                    {
+                        Username = authentication.Username,
+                        Password = authentication.Password
+                    };
+                }
             }
 
             Log.Info(string.Format("Retrieving git info from url '{0}'", repositoryUrl));