[FIX] expression: `child_of` domain with `parent_store` & non-existing parent

In the `product.template` model,
when searching products within a category that doesn't exist,
all products were returned, while none should be returned.

For instance,
In Sales > Products,
In the search input, search with internal category:
"A category that doesn't exist",
all products were returned.

opw-649548

openerp/osv/expression.py

@@ -729,6 +729,8 @@ class expression(object):
             """ Return a domain implementing the child_of operator for [(left,child_of,ids)],
                 either as a range using the parent_left/right tree lookup fields
                 (when available), or as an expanded [(left,in,child_ids)] """
+            if not ids:
+                return FALSE_DOMAIN
             if left_model._parent_store and (not left_model.pool._init):
                 # TODO: Improve where joins are implemented for many with '.', replace by:
                 # doms += ['&',(prefix+'.parent_left','<',o.parent_right),(prefix+'.parent_left','>=',o.parent_left)]