[#137] metabase: Do not select objects if there is no filterable attribute

In previous metabase implementation the absence of an attribute presented in
the search filter did not exclude the object from the result. Change this
behavior to exclude the object from the result.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/metabase/select.go

@@ -32,7 +32,8 @@ func (db *DB) Select(fs object.SearchFilters) ([]*object.Address, error) {
 			// get bucket with values
 			keyBucket := indexBucket.Bucket([]byte(key))
 			if keyBucket == nil {
-				continue
+				// no object has this attribute => empty result
+				return nil
 			}
 
 			fVal := f.Value()