Remove nullable parameter from createNamedBuilder call

The call to createNamedBuilder in EventController.php and ExportController.php was originally passing null as a parameter. This was generating PHPStan warnings. This commit replaces null with an empty string to comply with the method's expected types.

phpstan-baseline-level-5.neon

@@ -100,11 +100,6 @@ parameters:
 			count: 1
 			path: src/Bundle/ChillEventBundle/Controller/EventController.php
 
-		-
-			message: "#^Parameter \\#1 \\$name of method Symfony\\\\Component\\\\Form\\\\FormFactoryInterface\\:\\:createNamedBuilder\\(\\) expects string, null given\\.$#"
-			count: 1
-			path: src/Bundle/ChillEventBundle/Controller/EventController.php
-
 		-
 			message: "#^Parameter \\#1 \\$value of function count expects array\\|Countable, Chill\\\\MainBundle\\\\Entity\\\\Center given\\.$#"
 			count: 1
@@ -165,11 +160,6 @@ parameters:
 			count: 1
 			path: src/Bundle/ChillMainBundle/Controller/ExportController.php
 
-		-
-			message: "#^Parameter \\#1 \\$name of method Symfony\\\\Component\\\\Form\\\\FormFactoryInterface\\:\\:createNamedBuilder\\(\\) expects string, null given\\.$#"
-			count: 1
-			path: src/Bundle/ChillMainBundle/Controller/ExportController.php
-
 		-
 			message: "#^Parameter \\#3 \\$alias of method Chill\\\\MainBundle\\\\Controller\\\\ExportController\\:\\:exportFormStep\\(\\) expects string, Symfony\\\\Component\\\\HttpFoundation\\\\Request given\\.$#"
 			count: 1

src/Bundle/ChillMainBundle/Controller/ExportController.php

@@ -288,7 +288,7 @@ class ExportController extends AbstractController
 
         $builder = $this->formFactory
             ->createNamedBuilder(
-                null,
+                '',
                 FormType::class,
                 $defaultFormData,
                 [