Improve DOM extension types

[shira-374]

#1951

• DOMNode::insertBefore() accepts null in place of $refnode
• DOMXPath::evaluate() and DOMXPath::query() accept null in place of $contextnode
• DOMDocument::$ownerDocument is always null
• DOMNode::$attributes is null unless the node is instance of DOMElement
• DOMNode::$parentNode is null for root nodes
• DOMNode::$firstChild, $lastChild, $previousSibling, $nextSibling can be null if there is no such node
• DOMNode::$namespaceURI, $baseURI can be null if they aren't defined for the document
• DOMNode::$prefix seems to always be a string contrary to what the PHP docs say

[shira-374]

I guess it might be better to assume that parentNode won't be null (unless Psalm knows that it's a DOMDocument instance).

[muglug]

I guess it might be better to assume that parentNode won't be null (unless Psalm knows that it's a DOMDocument instance).

I think it's fine - if there are complaints I can allow Psalm to not care about null there.

[shira-374]

I think it's fine - if there are complaints I can allow Psalm to not care about null there.

I've already reverted it via fixup before the merge 🙈 Should I send another PR or will you change it? test-with-real-projects will not pass though.