HPCC-8335 Option on a field to control the default value

The default value of a field will be used:

1. When a dictionary lookup does not find a match
2. When a dataset select indexes a non-existent row
3. When assigning SELF := [] in a transform
4. In the default row passed to transforms by a OUTER join
5. When reading from XML and a column is not present

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>