[timer] Handle cancel(nil) as NOP.

timer.lua

@@ -88,7 +88,9 @@ function Timer:every(delay, after, count)
 end
 
 function Timer:cancel(handle)
-    self.functions[handle] = nil
+    if handle then
+        self.functions[handle] = nil
+    end
 end
 
 function Timer:clear()