add(update_callback) allow nil callback

Suggested by @sclu1034

lib/wibox/widget/template.lua

@@ -16,10 +16,6 @@ local wbase = require("wibox.widget.base")
 local gtable = require("gears.table")
 local gtimer = require("gears.timer")
 
-local default_update_callback = function()
-    return nil
-end
-
 local default_template_widget = wbase.empty_widget()
 
 local template = {
@@ -28,7 +24,9 @@ local template = {
 }
 
 function template:_do_update_now(args)
+    if type(self.update_callback) == 'function' then
         self:update_callback(args)
+    end
     template.queued_updates[self] = false
 end
 
@@ -73,7 +71,7 @@ function template.new(args)
 
     local widget = wbase.make_widget_from_value(widget_template)
 
-    widget.update_callback = args.update_callback or default_update_callback
+    widget.update_callback = args.update_callback
     widget.update_args = args.update_args or {}
 
     gtable.crush(widget, template, true)

spec/wibox/widget/template_spec.lua

@@ -32,7 +32,7 @@ describe("wibox.widget.template", function()
         it("batch calls", function()
             local spied_update_callback = spy.new(function() end)
 
-            widget.update_callback = spied_update_callback
+            widget.update_callback = function(...) spied_update_callback(...) end
 
             -- Multiple calls to update
             widget:update()
@@ -54,7 +54,7 @@ describe("wibox.widget.template", function()
             local update_args = { foo = "bar" }
 
             widget.update_args = args_structure
-            widget.update_callback = spied_update_callback
+            widget.update_callback = function(...) spied_update_callback(...) end
 
             widget:update(update_args)