message.js should work with whitespace in empty message wrappers

Problem/Motivation

Found in #3406612: Exceptions in batch no longer are shown on the page: Javascript error where we had to use olivero as the test them because the message.js does not work with the stark theme.

This is because core/modules/system/templates/status-messages.html.twig does not print anything under the top level div if there are no messages.

in core/misc/message.js we have

return wrapper.innerHTML === ''
        ? Drupal.Message.messageInternalWrapper(wrapper)
        : wrapper.firstElementChild;

But wrapper.innerHTML !== '' because there is return character. Any other character would also make it break. So in this case wrapper.firstElementChild will be NULL.

We could change the template but any contrib or custom template with this would also break.

Steps to reproduce

• Using the stark theme (or many others)
• On a fresh page load, run (new Drupal.Message()).clear(); in the browsers JS console
• The code will go this.messageWrapper.querySelectorAll, but messageWrapper will be null

Proposed resolution

Check wrapper.childElementCount === 0 instead of wrapper.innerHTML === ''

Remaining tasks

Review