Avoid importing dummy empty paragraphs with class "auto-cursor-target"

Confluence exports are riddled with codes like <p class="auto-cursor-target"><br /></p>

We shall avoid importing them. It looks like an artifact of their WYSIWYG editor.

See https://community.atlassian.com/t5/Confluence-questions/Why-does-Confluence-use-class-quot-auto-cursor-target-quot/qaq-p/1119238

When handling this, we should be careful that paragraphs with class auto-cursor-target are sometime not empty and contain useful content.

See also https://github.com/xwikisas/xwiki-pro-macros/issues/217