fix(Sdk): Made sure not to throw when not able to resolve an action type

Author: cdavernas

src/ServerlessWorkflow.Sdk/Models/ActionDefinition.cs

@@ -49,7 +49,7 @@ public virtual string Type
             else if (this.Subflow != null)
                 return ActionType.Subflow;
             else
-                throw new InvalidOperationException("Failed to determine the action type");
+                return string.Empty;
         }
     }