allow null inside array to be converted to xml

[tsuzukit]

Regarding this issue. https://github.com/spatie/array-to-xml/issues/169

This will allow null to be present in array.

For example, convert this array

$arr = [
    'test' => null,
];

ArrayToXml::convert($arr);

to below xml

<?xml version="1.0"?>
<root>
  <test/>
</root>

[tsuzukid]

Hi @freekmurze or someone in charge of merging PR

Sorry to rush, but it would be great if you can take a look at this PR.
If there is any concern, I am happy to fix!

Thanx!

[freekmurze]

Thanks!