$fixVersion default value is unnecessary set to empty array unlike any other property

In IssueField.php, line 50, change the:

```php
public array $fixVersions = [];
```

to:

```php
public ?array $fixVersions = null;
```

This change sets the default value to null, aligning it with other fields. It helps prevent unexpected behavior that might occur when this field is unintentionally set to an empty array.


