Fix for Action Masking when using Continuous control (#1116)

unity-environment/Assets/ML-Agents/Scripts/ActionMasker.cs

         /// actions.</returns>
         public bool[] GetMask()
         {
-            AssertMask();
+            if (_currentMask != null)
+            {
+                AssertMask();
+            }
             return _currentMask;
         }