small fix in html converter, although there was no image, e.g., in a questionlist, the <img> tag was inserted

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/exams@627 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
still trying to get qti 2.1 run

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/exams@628 edb9625f-4e0d-4859-8d74-9fd3b1da38cb