fix(w.w.template) draw

The draw method exists only for widgets. When dealing with more complex
widget, this method is not necessary implemented (i.e. with layouts).

We need to check the method is defined by the child before calling it.

lib/wibox/widget/template.lua

@@ -40,7 +40,9 @@ function template:fit(context, width, height)
 end
 
 function template:draw(...)
-    return self._private.widget:draw(...)
+    if type(self._private.widget.draw) == "function" then
+        return self._private.widget:draw(...)
+    end
 end
 
 function template:_do_update_now()