plugin text syntaxes: fix empty string handling in _remove_markups.

libervia/backend/plugins/plugin_misc_text_syntaxes.py

@@ -475,5 +475,7 @@ class TextSyntaxes(object):
         @return: the cleaned string
         """
         cleaner = clean.Cleaner(kill_tags=["style"])
+        if not xhtml:
+            return ""
         cleaned = cleaner.clean_html(html.fromstring(xhtml))
         return html.tostring(cleaned, encoding=str, method="text")