Composite table keys with non-existent XPATH target now Null

When specifying a composite table key it has been observed that some of the values of the key may not exist.  This is seen in chassis hardware.
In the event of an unfound XPATH the key value in the tuple is now None
Original behaviour was an uncaught exception.
Merge pull request #298 from shermdog/table-missing-xpath

Composite table keys with non-existent XPATH target now Null