fix undefined format_to_use_for_docs (#632)

* fix undefined format_to_use_for_docs

* fix undefined format_to_use_for_docs

Author: DarkaOnLine

src/Http/Controllers/SwaggerController.php

@@ -40,7 +40,9 @@ public function docs(Request $request)
         $fileSystem = new Filesystem();
         $documentation = $request->offsetGet('documentation');
         $config = $request->offsetGet('config');
-        $yamlFormat = ($config['paths']['format_to_use_for_docs'] === 'yaml');
+
+        $formatToUseForDocs = $config['paths']['format_to_use_for_docs'] ?? 'json';
+        $yamlFormat = ($formatToUseForDocs === 'yaml');
 
         $filePath = sprintf(
             '%s/%s',