FIX: fix theme:set command.

This command did not set theme properly, when THEME was in .env file, but with no value (THEME= only - as default in .env.example).

config/app.php

@@ -37,7 +37,7 @@ return [
     |
     */
 
-    'url'             => 'http://localhost',
+    'url'             => env('SITE_URL', 'http://localhost'),
     /*
     |--------------------------------------------------------------------------
     | Application Timezone
@@ -49,7 +49,7 @@ return [
     |
     */
 
-    'timezone'        => 'UTC',
+    'timezone'        => env('TIMEZONE', 'UTC'),
     /*
     |--------------------------------------------------------------------------
     | Application Locale Configuration
@@ -61,7 +61,7 @@ return [
     |
     */
 
-    'locale'          => 'en',
+    'locale'          => env('LOCALES', 'en'),
     /*
     |--------------------------------------------------------------------------
     | Application Fallback Locale
@@ -97,7 +97,7 @@ return [
      */
     'upload_url'      => env('UPLOAD_URL', PHP_SAPI === 'cli' ? '' : url('uploads')),
     'thumbnail_url'   => env('THUMBNAIL_URL', PHP_SAPI === 'cli' ? '' : url('uploads/thumbnails')),
-    'theme'           => env('THEME', 'default_theme'),
+    'theme'           => env('THEME'),
     /*
     |--------------------------------------------------------------------------
     | Logging Configuration