JavaScript: fix .offsetHeight for empty <div>.

git-svn-id: svn://svn.code.sf.net/p/htmlunit/code/trunk@9360 5f5364db-9458-4db8-a492-e30667be6df6
Fixing build.

git-svn-id: svn://svn.code.sf.net/p/htmlunit/code/trunk@9361 5f5364db-9458-4db8-a492-e30667be6df6