bugfix: fixed keyword argument `format` -> `format_` bug

introduced in 22561a03dee1ec61c12ae4d62b054e57505dc000
bugfix: ignore empty namespace in Selector/Index/XPath configs

Don't add default namespace (i.e. prefix == None) to namespace mapping
for Selectors/XPaths, or those implicitly created by Index configs.
These cause errors when selecting data from records.