parentFolderPlaceholder default value in Settings implementation is inconsistent with the javadoc

In relation to wicket-1992,
the javadoc for:
org.apache.wicket.settings.IResourceSettings#setParentFolderPlaceholder
says:
"Note that by default the placeholder string is empty '' and thus will not allow to access parent folders."

But the default is null, not ''
and this generates src url like this: resources/com.test.sub.pack.TestPage/null/null/base/information.png
which do work, but probably shouldn't.