Fix for fatal errors caused by incorrect closing tags in HTML, such as `</br>`. In these cases, the `get_attribute_names_with_prefix` method of the `WP_HTML_Tag_Processor` returns `null`, and the Interactivity API was not handling this situation correctly.
Props hugosolar, jonsurrell, darerodz.
Fixes#63891.
Built from https://develop.svn.wordpress.org/trunk@61197
git-svn-id: http://core.svn.wordpress.org/trunk@60533 1a063a9b-81f0-0310-95a4-ce76da25c4cd