Fall back to empty siblings list if no parentNode is present

[julianrubisch]

Bug Fix

Description

When combining updates_for with a page or selector morph, a race condition could occur where SR's elementToXPath would try to parse a document fragment that's not yet connected to the DOM as part of updates_for's morphing process. Traversing up the tree, such an element would have no parentNode and thus no siblings.

This PR fixes it by falling back to an empty siblings array should that occur.

Checklist

• My code follows the style guidelines of this project
• Checks (StandardRB & Prettier-Standard) are passing
• This is not a documentation update