Merge branch '3.4' into 4.3

* 3.4:
  Fix inconsistent return points.
  [Security/Core] UserInterface::getPassword() can return null
  [Router] Fix TraceableUrlMatcher behaviour with trailing slash

Author: nicolas-grekas

Definition/Dumper/XmlReferenceDumper.php

@@ -296,5 +296,7 @@ private function writeValue($value): string
         if (\is_array($value)) {
             return implode(',', $value);
         }
+
+        return '';
     }
 }

Loader/FileLoader.php

@@ -166,5 +166,7 @@ private function doImport($resource, $type = null, bool $ignoreErrors = false, $
                 throw new LoaderLoadException($resource, $sourceResource, null, $e, $type);
             }
         }
+
+        return null;
     }
 }