If an element is returnable by xmltable, but marked as nil more than likely an exception will be produced when attempting to convert the empty string value to a result.

For example:

select * from xmltable('/a/b' passing convert('<a xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><b xsi:nil="true"/><b>1</b></a>', xml) columns val integer path '.') as x

will error converting the first b element to an integer.