chore: use nullish coalescing operator for arcjet fallback

Author: ixartz

src/libs/Arcjet.ts

@@ -4,7 +4,7 @@ import arcjet, { shield } from '@arcjet/next';
 export default arcjet({
   // Get your site key from https://launch.arcjet.com/Q6eLbRE
   // Use `process.env` instead of Env to reduce bundle size in middleware
-  key: process.env.ARCJET_KEY || '',
+  key: process.env.ARCJET_KEY ?? '',
   // Identify the user by their IP address
   characteristics: ['ip.src'],
   rules: [