[12.x] Fix type casting for environment variables in config files (#6637)

* Update app.php

* Update cache.php

* Update database.php

* Update logging.php

* Update mail.php

config/app.php

@@ -101,7 +101,7 @@ return [
 
     'previous_keys' => [
         ...array_filter(
-            explode(',', env('APP_PREVIOUS_KEYS', ''))
+            explode(',', (string) env('APP_PREVIOUS_KEYS', ''))
         ),
     ],