Add check header and footer not null and exist before unlink

Header & Footer file now only removed when option is not null and file exists.

Fixes:
when option['header-html'] or option['footer-html'] is null then
unlink(): No such file or directory
thrown on generate.
fix CS

```php
if (isset($options['header-html'])) {
    return !$this->isOptionUrl($options['header-html']);
}

return false;
```