Infer more DOMDocument types when loaded

[jnvsor]

https://psalm.dev/r/01eac6a01b

When DOMDocument methods load, loadXML, loadHTML, loadHTMLFile return true DOMDocument must have at least one node.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/01eac6a01b

<?php

function test(string $file): DOMNode {
    $xml = new DOMDocument();
    $check = $xml->loadXML($file, LIBXML_NOWARNING | LIBXML_NOERROR);

    if (false === $check) {
        throw new Exception();
    }
    
    return $xml->firstChild;
}

Psalm output (using commit 03ee02c):

ERROR: NullableReturnStatement - 11:12 - The declared return type 'DOMNode' for test is not nullable, but the function returns 'DOMNode|null'

ERROR: InvalidNullableReturnType - 3:30 - The declared return type 'DOMNode' for test is not nullable, but 'DOMNode|null' contains null