fix: ccp_clean_html handle case if removing ccp styling results in empty string

[recursivetree]

ccp_clean_html converts the messages of evemails into browser-presentable html. The function currently crashes if it is called with u'' or a few other, specific messages.

In the implementation, DOMDocument->loadHTML doesn't like empty strings. While we do have an empty string check at the top of the function, we apply transformations that can result in an empty string after the check, so it is possible that an empty string reaches DOMDocument->loadHTML.