->nullable()

Seems that the sql produced puts null in a string
```sql
 ALTER TABLE table CHANGE column column ENUM('Option 1','Option 2') DEFAULT 'NULL' COLLATE `utf8mb4_unicode_ci`;
```

Minor issue as resolved by adding `default(null)`

```php
$table->enum('column', ['Option 1', 'Option 2'])->nullable()->default(null)->change();
```