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

Change-Id: Ic738dbbee4ce972ebf7efcfda33ccbb3f3ac48ff
Reviewed-on: https://gerrit.libreoffice.org/11432
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>