exliff export: pass null values as empty string

Author: solverat

UPGRADE.md

@@ -1,5 +1,8 @@
 # Upgrade Notes
 
+## 3.2.2
+- [BUGFIX] Xliff Export: pass null values as empty string
+
 ## 3.2.1
 - [BUGFIX] Check published state for objects
 

src/EventListener/Admin/XliffListener.php

@@ -59,7 +59,7 @@ public function export(TranslationXliffEvent $event): void
                 $attributeSet->addAttribute(
                     self::XLIFF_TYPE,
                     sprintf('%s#%s', $integrator, $property),
-                    $value,
+                    $value ?? '',
                     false,
                     []
                 );