Fix empty string fallback for comment_package_manager

When comment_package_manager input is not set, core.getInput() returns
an empty string. Using nullish coalescing (??) fails to fall back to
the auto-detected packageManager because empty string is not nullish.
This causes the install command to render as 'i -g' instead of 'npm i -g'.

Switch to logical OR (||) so empty string correctly falls through to
the detected package manager.

Co-authored-by: Craig Martin <craig.martin@shopify.com>

Author: River (AI)

dist/index.js

@@ -214,7 +214,7 @@ try {
       : `Your snapshot${multiple ? 's have' : ' has'} been published to npm.**\n\n`;
 
     const messagePackageManager =
-      commentPackageManager?.toLowerCase() ?? packageManager;
+      commentPackageManager?.toLowerCase() || packageManager;
     let globalInstallMessage =
       messagePackageManager === 'yarn'
         ? 'yarn global add'