Do not enforce with empty string

Author: rullzer

lib/Migration/Version00005Date20200428123958.php

@@ -26,7 +26,6 @@ public function changeSchema(IOutput $output, Closure $schemaClosure, array $opt
 
 		$table = $schema->getTable('user_oidc');
 		$table->addColumn('display_name', 'string', [
-			'notnull' => true,
 			'length' => 255,
 			'default' => '',
 		]);