actions: raw: Actually make offset optional
actions: raw: Actually make offset optional

The documentation suggests that offset should be optional; in practice
offset isn't optional and the action will return an error out if unset:

    Action `` failed at stage Run, error: Couldn't parse offset strconv.ParseInt: parsing "": invalid syntax

So set the offset by default to 0 then only attempt to parse the parameter
from the YAML if it is explicitly set.

Fixes: b792b4722acb ("Add a raw action")

Signed-off-by: Christopher Obbard <0becaf7888cff31ba54e40b1ffb8263e09b8b7da@collabora.com>

go-debos/debos