Don't force users to match existing config

If an object is in the registry already, and no bespoke config has been
specified - don't consider that an exceptional case.

src/Core/ObjectRegistry.php

@@ -116,6 +116,11 @@ protected function _checkDuplicate($name, $config) {
 		}
 		$existingConfig = $existing->config();
 		unset($config['enabled'], $existingConfig['enabled']);
+
+		if (empty($config)) {
+			return;
+		}
+
 		if ($hasConfig && json_encode($config) !== json_encode($existingConfig)) {
 			$msg .= ' with the following config: ';
 			$msg .= var_export($existingConfig, true);