Revert new rules on list(NULL) assignment

[MichaelChirico]

Reverts #6167 (retaining its tests). Closes #6740. Re-opens #5558.

cc @joshhwuu

[github-actions]

Generated via commit b9436f4

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	4 minutes and 38 seconds
Installing different package versions	8 minutes and 16 seconds
Running and plotting the test cases	2 minutes and 22 seconds

[tdhock]

great, thanks.

[MichaelChirico]

Failed tests are because I missed one line in R/data.table.R. I should have done the reversion more properly like so:

git revert 1aa92bc4ab3206b5e9f8accdc5b205edbb76c73a
# resolve conflicts
git commit -a

# restore changes to tests
git checkout 1aa92bc4ab3206b5e9f8accdc5b205edbb76c73a -- inst/tests/tests.Rraw
# resolve conflicts ?
git commit -a

I discovered the missing line by doing the above in a local branch and then checking git diff against this branch.