fix warning (#59)

* fix warning

* allow infection plugin

Author: nick-zh

composer.json

@@ -34,5 +34,10 @@
     "branch-alias": {
       "dev-main": "3.0-dev"
     }
+  },
+  "config": {
+    "allow-plugins": {
+      "infection/extension-installer": true
+    }
   }
 }

src/Registry/SchemaRegistry.php

@@ -135,7 +135,11 @@ private function registerSchemaFile(\SplFileInfo $fileInfo): void
         }
 
         $schemaData = json_decode($fileContent, true, JSON_THROW_ON_ERROR);
-        $namespace = $schemaData['namespace'] ?? '';
+	$namespace = '';
+
+	if (true === isset($schemaData['namespace'])) {
+	    $namespace = $schemaData['namespace'];
+	}
 
         if (null === $schemaData) {
             throw new SchemaRegistryException(sprintf(SchemaRegistryException::FILE_INVALID, $fileName));