fix passing null value to preg_split (#507)

Author: solverat

UPGRADE.md

@@ -7,6 +7,7 @@
 - **[IMPROVEMENT]** Doctrine ORM 3.0 Support [#503](https://github.com/dachcom-digital/pimcore-formbuilder/pull/503)
 - **[BUGFIX]** API Channel: Keep array index when merging child nodes [@simon-matt-oetztal](https://github.com/dachcom-digital/pimcore-formbuilder/pull/496)
 - **[BUGFIX]** Update Custom Channel Documentation [#493](https://github.com/dachcom-digital/pimcore-formbuilder/issues/493)
+- **[BUGFIX]** Fix passing null value to `preg_split` 
 
 ## 5.1.4
 - **[BUGFIX]** Allow using double-opt-in variables in placeholder processor

src/OutputWorkflow/Channel/Email/Parser/MailParser.php

@@ -227,8 +227,8 @@ protected function extractPlaceHolder(string $str, array $fieldValues): mixed
             $str = str_replace($matches[0][$key], $value, $str);
         }
 
-        //remove invalid commas
-        $fragments = preg_split('@,@', $str, null, PREG_SPLIT_NO_EMPTY);
+        // remove invalid commas
+        $fragments = preg_split('@,@', $str, -1, PREG_SPLIT_NO_EMPTY);
         $fragmentsGlued = is_array($fragments) ? implode(',', $fragments) : $fragments;
         $extractedValue = is_string($fragmentsGlued) ? trim($fragmentsGlued) : $fragmentsGlued;