big_pipe JS checkMutation can throw type errors if parentNode is NULL

Problem/Motivation

In #3390178: big_pipe sometimes fails to load blocks we added checkMutation(node.parentNode) without checking if node.parentNode wasn't NULL.

It can be NULL in certain circumstances

Steps to reproduce

Unsure, this is only reproducible for me on circle CI (when upgrading from 10.2.0 to 10.2.2) so assume it's something with the page/JS speed during tests.

Full stack trace with aggregation disabled

TypeError: Cannot read properties of null (reading 'nodeType')
    at checkMutation (http://127.0.0.1:8080/core/modules/big_pipe/js/big_pipe.js?v=10.2.2:92:12)
    at checkMutationAndProcess (http://127.0.0.1:8080/core/modules/big_pipe/js/big_pipe.js?v=10.2.2:116:14)
    at NodeList.forEach (<anonymous>)
    at http://127.0.0.1:8080/core/modules/big_pipe/js/big_pipe.js?v=10.2.2:129:18
    at Array.forEach (<anonymous>)
    at MutationObserver.processMutations (http://127.0.0.1:8080/core/modules/big_pipe/js/big_pipe.js?v=10.2.2:128:15)

This is during a PHP JS test (using drupal-test-traits with the selenium2 driver), captured using the js_testing_log_test module.

Proposed resolution

Check node.parentNode

Merge request link

https://git.drupalcode.org/project/drupal/-/merge_requests/6358

Remaining tasks

Figure out how to test if it is testable.

User interface changes

N/a

API changes

N/a

Data model changes

N/a

Release notes snippet

N/a