Fixing mouse movement in .NET

The previous commit to the mouse movement for .NET changed the optional
element ID from `null` to the empty string. This is incorrect. If no
element is specified in the move (for non-spec-compliant driver), omit the
ID altogether.

dotnet/src/webdriver/Remote/RemoteMouse.cs

@@ -108,10 +108,6 @@ public void MouseMove(ICoordinates where, int offsetX, int offsetY)
                 string elementId = where.AuxiliaryLocator.ToString();
                 parameters.Add("element", elementId);
             }
-            else
-            {
-                parameters.Add("element", string.Empty);
-            }
 
             parameters.Add("xoffset", offsetX);
             parameters.Add("yoffset", offsetY);