[FIX] web_editor: properly merge a LI with text with a LI with text in P

A list item with text can be written as <li>text</li> or as
<li><p>text</p></li>. Issues occurred when both syntaxes need to be
merged together. Namely, the <p>text</p> was being appended to the other
list item, thereby creating a visible new line.