Fix EZP-21336: In IE8, every publish adds another empty paragraph at the end

JIRA: https://jira.ez.no/browse/EZP-21336

# Description

In IE8 in some conditions, the last *empty* paragraph added to avoid some issues in TinyMCE is not removed before storing the eZXML. As a result, the XML block is growing on each publish operation.
I was not able to reproduce this issue on my system, I suspect this is also related to a very specific version of IE8 and/or PHP and its linked libraries.

# Tests

manual tests on my system (no regression) + test by the 2nd line