Merge branch '3.4' into 4.4

* 3.4:
  Remove branch-version (keep them for contracts only)
  [Serializer] fix decoding float XML attributes starting with 0
  add missing dutch translations
  [Validator] Add missing romanian translations
  do not translate null placeholders or titles

Author: nicolas-grekas

composer.json

@@ -34,8 +34,5 @@
         "symfony/monolog-bridge": "For using the log server.",
         "symfony/expression-language": "For using the filter option of the log server."
     },
-    "minimum-stability": "dev",
-    "extra": {
-        "branch-version": "4.4"
-    }
+    "minimum-stability": "dev"
 }