Added null check for workspaceId in Assistant V1 Message.

Author: hmiri

Scripts/Services/Assistant/v1/Assistant.cs

@@ -132,6 +132,8 @@ public bool Message(SuccessCallback<object> successCallback, FailCallback failCa
                 throw new ArgumentNullException("successCallback");
             if (failCallback == null)
                 throw new ArgumentNullException("failCallback");
+            if (string.IsNullOrEmpty(workspaceId))
+                throw new ArgumentNullException("workspaceId");
 
             IDictionary<string, string> requestDict = new Dictionary<string, string>();
             if (request.Context != null)