fix(purify): Remove unnecessary conditional expressions

[ssi02014]

Summary

Hello 👋, @cure53

Node property "content" is not currently available.
There's no need to conditionally check it, as it always returns undefined.

currentNode.content; // Always undefined
currentNode.content.firstElementChild; // Always undefined

(!_isNode(currentNode.content) || !_isNode(currentNode.content.firstElementChild)) // Always true

I went through all the NodeType listed in the documentation below.
https://developer.mozilla.org/en-US/docs/Web/API/Node/nodeType

After fixing the code, the tests all passed, and I think it's safe to remove it from 3.x.

Tasks

Remove unnecessary conditional expressions

[cure53]

This should be safe, let me do some checks and merge, thanks 🙂