Fix installing on Oracle

Empty strings are stored as null in Oracle,
so a column with NotNull can not have an empty string as default

Signed-off-by: Joas Schilling <coding@schilljs.com>

core/Migrations/Version18000Date20190920085628.php

@@ -59,7 +59,9 @@ class Version18000Date20190920085628 extends SimpleMigrationStep {
 			$table->addColumn('displayname', Type::STRING, [
 				'notnull' => true,
 				'length' => 255,
-				'default' => '',
+				// Will be overwritten in postSchemaChange, but Oracle can not save
+				// empty strings in notnull columns
+				'default' => 'name',
 			]);
 		}