Merge branch '7.2' into 7.3

* 7.2:
  Fix typo
  [VarExporter] Dump implicit-nullable types as explicit to prevent the corresponding deprecation
  Allow mixed root on CompoundConstraintTestCase validator
  [Serializer] Fix readonly property initialization from incorrect scope
  Update BrevoRequestParser.php

Author: nicolas-grekas

ProxyHelper.php

@@ -633,7 +633,9 @@ public static function exportType(\ReflectionFunctionAbstract|\ReflectionPropert
             return '';
         }
         if (null === $glue) {
-            return (!$noBuiltin && $type->allowsNull() && !\in_array($name, ['mixed', 'null'], true) ? '?' : '').$types[0];
+            $defaultNull = $owner instanceof \ReflectionParameter && 'NULL' === rtrim(substr(explode('$'.$owner->name.' = ', (string) $owner, 2)[1] ?? '', 0, -2));
+
+            return (!$noBuiltin && ($type->allowsNull() || $defaultNull) && !\in_array($name, ['mixed', 'null'], true) ? '?' : '').$types[0];
         }
         sort($types);