DX: apply rector rulesset up to PHP72

src/Bundle/ChillMainBundle/Export/Formatter/SpreadSheetFormatter.php

@@ -435,7 +435,7 @@ class SpreadSheetFormatter implements FormatterInterface
      * Get the displayable result.
      *
      * @param string $key
-     * @param string $value
+     * @param mixed $value
      *
      * @return string
      */
@@ -445,7 +445,7 @@ class SpreadSheetFormatter implements FormatterInterface
             $this->initializeCache($key);
         }
 
-        $value = null === $value ? '' : $value;
+        $value = $value ?? '';
 
         return call_user_func($this->cacheDisplayableResult[$key], $value);
     }