Address review: simplify template branch, add constructor comment

- Remove unnecessary TriggerForwarder.template() branch —
  TemplateArguments() already handles the empty case.
- Add comment on Automation default constructor explaining
  its purpose for the forwarder pattern.

esphome/automation.py

@@ -700,9 +700,5 @@ async def build_callback_automation(
     # one operator() per forwarder type; different automation pointers are just
     # data in the struct.
     if forwarder is None:
-        forwarder = (
-            TriggerForwarder.template(templ)
-            if arg_types
-            else TriggerForwarder.template()
-        )
+        forwarder = TriggerForwarder.template(templ)
     cg.add(getattr(parent, callback_method)(cg.RawExpression(f"{forwarder}{{{obj}}}")))

esphome/core/automation.h

@@ -470,6 +470,7 @@ template<typename... Ts> class ActionList {
 
 template<typename... Ts> class Automation {
  public:
+  /// Default constructor for use with TriggerForwarder (no Trigger object needed).
   Automation() = default;
   explicit Automation(Trigger<Ts...> *trigger) { trigger->set_automation_parent(this); }