BUG (CoW): fix reference tracking in replace_list with None

[jorisvandenbossche]

For a specific case in list_replace when replacing with a non-present value with None, we were returning the block itself instead of a shallow copy (as is done in other code paths in this function), messing up the reference tracking (we are removing the block itself to avoid multiple copies when replacing multiple values).

• closes BUG: replace with 3 or more None values fails when copy_on_write is enabled #59770
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[phofl]

thx!

[jorisvandenbossche]

@meeseeksdev backport to 2.3.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout 2.3.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 3e8ac12d1dacc2308b2f4c2869fa7bc2079bd323

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #59807: BUG (CoW): fix reference tracking in replace_list with None'

4. Push to a named branch:

git push YOURFORK 2.3.x:auto-backport-of-pr-59807-on-2.3.x

5. Create a PR against branch 2.3.x, I would have named this PR:

"Backport PR #59807 on branch 2.3.x (BUG (CoW): fix reference tracking in replace_list with None)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.

[jorisvandenbossche]

Manual backport at #59943