Steps to reproduce the issue

Go to the TinyMCE plugin and add a Language of Parts item but leave the fields blank.
Save and go to edit an article.

Expected result

Article Edit Form

Actual result

Error page:
array_merge(): Argument #1 must be of type array, null given
JROOT/plugins/editors/tinymce/src/PluginTraits/DisplayTrait.php:393

System information (as much as possible)

Additional comments

Discovered by a user who accidentally added an empty field.