hawkbit-upload: Backport fix check_error() preventing normal upload to v2020

By default jq -r returns "null" string if the property does not exist. Replacing "null" by empty using the alternative operator allows to keep normal bash behavior, see https://github.com/stedolan/jq/issues/354https://github.com/stedolan/jq/issues/354#issuecomment-43147898.

The first commit fixes indentation.

Signed-off-by: Frédéric Danis frederic.danis@collabora.com