Merge pull request #1032 from nextcloud/fix-migration-notnull

fix migration by allowing null values for 'nid'

lib/Migration/Version0003Date20200823023911.php

@@ -139,6 +139,7 @@ class Version0003Date20200823023911 extends SimpleMigrationStep {
 			[
 				'length'   => 11,
 				'unsigned' => true,
+				'notnull' => false,
 			]
 		);
 	}