Fix null reference error when passing a null array to an action segment. (#4358)

com.unity.ml-agents/Runtime/Actuators/ActionSegment.cs

         /// be set to 0 and the <see cref="Length"/> will be set to `actionArray.Length`.
         /// </summary>
         /// <param name="actionArray">The action array to use for the this segment.</param>
-        public ActionSegment(T[] actionArray) : this(actionArray, 0, actionArray.Length) { }
+        public ActionSegment(T[] actionArray)
+            : this(actionArray ?? System.Array.Empty<T>(), 0, actionArray?.Length ?? 0) { }
 
         /// <summary>
         /// Construct an <see cref="ActionSegment{T}"/> with an underlying array
         public ActionSegment(T[] actionArray, int offset, int length)
         {
 #if DEBUG
-            CheckParameters(actionArray, offset, length);
+            CheckParameters(actionArray ?? System.Array.Empty<T>(), offset, length);
-            Array = actionArray;
+            Array = actionArray ?? System.Array.Empty<T>();
             Offset = offset;
             Length = length;
         }

com.unity.ml-agents/Tests/Editor/Actuators/ActionSegmentTests.cs

             }
         }
 
+        [Test]
+        public void TestNullConstructor()
+        {
+            var actionSegment = new ActionSegment<float>(null);
+            Assert.IsTrue(actionSegment.Length == 0);
+            Assert.IsTrue(actionSegment.Array == Array.Empty<float>());
+        }
+
     }
 
 }