mediawiki: use String.length()==0 instead of String.equals(empty string)

Change-Id: I849da46aee801686cf6a581d65276ab60575c078
Reviewed-on: https://gerrit.libreoffice.org/11372
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>