[rikvanriel]

items: fix +index crash on unmatched itemid

When the index view is rooted at an item addressed by something other
than a name, such as /+index/@itemid/... for an itemid that matches
nothing, the item has no NAME in its meta and thus no subitem prefixes.
build_index_query indexed subitem_prefixes[0] unconditionally and died
with IndexError instead of rendering an empty index -- unlike
get_subitem_revs, which already skips the prefix filter when the item
has no names.

Fix by returning a NullQuery matching nothing when there are no
prefixes; the view then renders an empty index as it does for a name
with no subitems.

The regression test roots an index at a nonexistent itemid and locks in
the empty result; it fails with IndexError on the old code.