Update tests to respect html() changes
What does this MR do?
This MR closes #191270 (closed)
When asserting for empty node - we're using isEmpty instead of html() to be undefined.
For spec/frontend/notes/components/discussion_notes_replies_wrapper_spec.js we're replacing html() with outerHTML - there is no way to disable pretty formatting of HTML in @vue/test-utils and we want this spec to be compatible both with beta.25 and beta.30 version