Prevent issues with Doctrine Inflector due to null string

Author: tobias-93

Resources/templates/CommonAdmin/ExcelAction/ExcelBuilderAction.php.twig

@@ -32,6 +32,7 @@ class ExcelController extends \{{ namespace_prefix }}\{{ bundle_name }}\Controll
         $this->request = $request;
 
         if ($key === null) {
+          $key = '';
           // Standard credential check
           {{ block('security_action') }}
         } else {
@@ -55,7 +56,7 @@ class ExcelController extends \{{ namespace_prefix }}\{{ bundle_name }}\Controll
           $suffix = Inflector::classify($key);
           if (!method_exists($this,"createSpreadsheetHeader$suffix")) {
               // back to defaults
-              $key = null;
+              $key = '';
               $suffix = '';
           }
 
@@ -100,7 +101,7 @@ class ExcelController extends \{{ namespace_prefix }}\{{ bundle_name }}\Controll
           $suffix = Inflector::classify($key);
           if (!method_exists($this,"createExcelHeader$suffix")) {
               // back to defaults
-              $key = null;
+              $key = '';
               $suffix = '';
           }