fix: return 400 error for initial payload

Author: alexdln

modules/isr-fallback.ts

@@ -31,7 +31,7 @@ export default defineNuxtModule({
           const outputPath = resolve(nitro.options.output.serverDir, '..', path, htmlFallback)
           mkdirSync(resolve(nitro.options.output.serverDir, '..', path), { recursive: true })
           writeFileSync(outputPath, spaTemplate)
-          writeFileSync(outputPath.replace(htmlFallback, jsonFallback), 'null')
+          writeFileSync(outputPath.replace(htmlFallback, jsonFallback), '{}')
         }
       })
     })

nuxt.config.ts

@@ -364,6 +364,7 @@ function getISRConfig(expirationSeconds: number, options: ISRConfigOptions = {})
         fallback:
           options.fallback === 'html' ? 'spa.prerender-fallback.html' : 'payload-fallback.json',
         initialHeaders: options.fallback === 'json' ? { 'content-type': 'application/json' } : {},
+        initialStatus: options.fallback === 'json' ? 400 : undefined,
       } as { expiration: number },
     }
   }