Fix inconsistent return points.

Author: derrabus

Definition/Dumper/XmlReferenceDumper.php

@@ -306,5 +306,7 @@ private function writeValue($value)
         if (\is_array($value)) {
             return implode(',', $value);
         }
+
+        return '';
     }
 }

Loader/FileLoader.php

@@ -168,5 +168,7 @@ private function doImport($resource, $type = null, $ignoreErrors = false, $sourc
                 throw new FileLoaderLoadException($resource, $sourceResource, null, $e, $type);
             }
         }
+
+        return null;
     }
 }