fix NullReferenceException on session run with ITensorOrOperation signature

src/TensorFlowNET.Core/Sessions/BaseSession.cs

@@ -65,7 +65,8 @@ namespace Tensorflow
 
         public virtual NDArray run(ITensorOrOperation fetche, params FeedItem[] feed_dict)
         {
-            return _run(fetche, feed_dict)[0];
+            var results = _run(fetche, feed_dict);
+            return fetche is Tensor ? results[0] : null;
         }
 
         public virtual (NDArray, NDArray, NDArray, NDArray, NDArray) run(

test/TensorFlowNET.UnitTest/SessionTest.cs

@@ -133,6 +133,17 @@ namespace TensorFlowNET.UnitTest
                 }
             }
         }
+        
+        [TestMethod]
+        public void Autocast_Case0()
+        {
+            var sess = tf.Session().as_default();
+            ITensorOrOperation operation = tf.global_variables_initializer();
+            // the cast to ITensorOrOperation is essential for the test of this method signature
+            var ret = sess.run(operation);
+
+            ret.Should().BeNull();
+        }        
 
         [TestMethod]
         public void Autocast_Case1()