job: properly initialize all enum fields

job: properly initialize all enum fields

This changes the .result field to invalid initially, which arguably
makes more sense than "done", which was previously the default.

This is a correctnes fix, and afaics has no effect on the API, since we
do not expose this 1:1 as D-Bus property: it's only seen on D-Bus as
part of the job completion signal, at which part it is correctly
initialized.

Noticed while reviewing: https://github.com/systemd/systemd/pull/41583