Allow to find an item without a selected element when using the type "do...

Hi,

This patch allows to find an item without a selected element when using the type "does not contain" in advanced search.

For example, if I need to find all items without "free" in the field "Rights", I use <code>DC:Rights / does not contains / free</code>. With current release, items without any right (the element is not filled) are not found, although these items answer to the request.

Daniel Berthereau
Infodoc & Knowledge management