refactor: follow upstream's suggestions on when to use EmptyGURL() (#45498)

- Prefer GURL() when we want to return a non-reference empty URL.

- In ServiceWorkerMain::GetStorageKey(), use a reference instead
  of instantiating a new temporary GURL.

From url/gurl.h:

> // Returns a reference to a singleton empty GURL. This object is for
> // callers who return references but don't have anything to return in
> // some cases. If you just want an empty URL for normal use, prefer
> // GURL().

shell/browser/extensions/api/management/electron_management_api_delegate.cc

@@ -214,7 +214,7 @@ GURL ElectronManagementAPIDelegate::GetEffectiveUpdateURL(
     const extensions::Extension& extension,
     content::BrowserContext* context) const {
   // TODO(codebytere): we do not currently support ExtensionManagement.
-  return GURL::EmptyGURL();
+  return {};
 }
 
 void ElectronManagementAPIDelegate::ShowMv2DeprecationReEnableDialog(