Unit test is failing on r-devel due to `is.atomic(NULL)` change

[cpsievert]

Problem can be seen here

https://github.com/rstudio/htmltools/actions/runs/6378121706/job/17308114511#step:4:122

And the notification from CRAN:

Dear maintainer,

Please see the problems shown on
<https://cran.r-project.org/web/checks/check_results_htmltools.html>.

This is from

r85227 | maechler | 2023-09-28 09:38:16 +0200 (Thu, 28 Sep 2023) | 1 line
is.atomic(NULL) now is FALSE

Please see
<https://stat.ethz.ch/pipermail/r-devel/2023-September/082892.html> for
more information and guidance.

Please correct before 2023-10-14 to safely retain your package on CRAN.