Don't set CACHE_PREFIX to empty string by default

[Jamesking56]

Setting CACHE_PREFIX to an empty string by default overrides the default behaviour of using APP_NAME in the cache config file:

/*
|--------------------------------------------------------------------------
| Cache Key Prefix
|--------------------------------------------------------------------------
|
| When utilizing the APC, database, memcached, Redis, and DynamoDB cache
| stores, there might be other applications using the same cache. For
| that reason, you may prefix every cache key to avoid collisions.
|
*/

'prefix' => env('CACHE_PREFIX', Str::slug(env('APP_NAME', 'laravel'), '_').'_cache_'),

This leads to unintended consequences if this has been blindly copied into the app's main .env file without setting a value.

Leaving this unset would ensure that at least the app name is being used for this value and that it is set by the framework.