[restart] Use a user provided default constructor for RestartButton (#19168)

esphome/components/restart/button/restart_button.h

@@ -7,6 +7,9 @@ namespace esphome::restart {
 
 class RestartButton final : public button::Button, public Component {
  public:
+  // User provided, not "= default": `new(p) RestartButton()` would zero-fill .bss that is already zero.
+  RestartButton() {}
+
   void dump_config() override;
 
  protected: