Merge branch '3.4' into 4.4

* 3.4:
  [appveyor] bump cache
  [DI] µfix

Author: nicolas-grekas

Loader/YamlFileLoader.php

@@ -345,7 +345,7 @@ private function parseDefinition(string $id, $service, string $file, array $defa
 
         if (isset($service['alias'])) {
             $this->container->setAlias($id, $alias = new Alias($service['alias']));
-            if (\array_key_exists('public', $service)) {
+            if (isset($service['public'])) {
                 $alias->setPublic($service['public']);
             } elseif (isset($defaults['public'])) {
                 $alias->setPublic($defaults['public']);