Add better error message for missing app key

src/Illuminate/Encryption/EncryptionServiceProvider.php

@@ -2,6 +2,7 @@
 
 namespace Illuminate\Encryption;
 
+use RuntimeException;
 use Illuminate\Support\Str;
 use Illuminate\Support\ServiceProvider;
 
@@ -24,6 +25,12 @@ public function register()
                 $key = base64_decode(substr($key, 7));
             }
 
+            if (is_null($key) || $key === '') {
+                throw new RuntimeException(
+                    'The application encryption key is missing. Run php artisan key:generate to generate it.'
+                );
+            }
+
             return new Encrypter($key, $config['cipher']);
         });
     }