TaskBase null fix

When this class is dervied and used with no task set as it's dependency.
It returns a false for success and should return a default value for
T

Author: StanleyGoldman

src/GitHub.Api/Tasks/TaskBase.cs

@@ -597,7 +597,7 @@ public TaskBase(CancellationToken token)
         {
             Task = new Task<TResult>(() =>
             {
-                var ret = RunWithData(DependsOn?.Successful ?? previousSuccess, DependsOn.Successful ? ((ITask<T>)DependsOn).Result : default(T));
+                var ret = RunWithData(DependsOn?.Successful ?? previousSuccess, (DependsOn?.Successful ?? false) ? ((ITask<T>)DependsOn).Result : default(T));
                 tcs.SetResult(ret);
                 AdjustNextTask(ret);
                 return ret;