contract: change default value of lastAttachment so server doesn't try to parse empty string

[orndorffgrant]

Why is this needed?

Machines attached with a pro version <=28.1 don't have a lastAttachment time. The fallback value of "" was trying to be parsed by the contract server, which resulted in errors. The new fallback value of None is treated as nil by the backend and isn't parsed.

Test Steps

env SHELL_BEFORE=1 ./tools/test-in-lxd.sh xenial
apt update
apt install ubuntu-advantage-tools
pro attach $token
exit # new version gets installed after exit and lxc shell is re-started
sudo pro disable esm-infra
# Before this PR: you'd get an error here
# With this PR: this should succeed

Checklist

• [n/a] I have updated or added any unit tests accordingly
• [n/a] I have updated or added any integration tests accordingly
• [n/a] Changes here need to be documented, and this was done in:

Does this PR require extra reviews?

• Yes
• No

[orndorffgrant]

@TeodorPt FYI

[github-actions]

Jira: This PR is not related to a Jira item. (The PR title does not include a SC-#### reference)

GitHub Issues: No GitHub issues are fixed by this PR. (No commits have Fixes: #### references)

Launchpad Bugs: No Launchpad bugs are fixed by this PR. (No commits have LP: #### references)

Documentation: The changes in this PR do not require documentation changes.

👍 this comment to confirm that this is correct.

[renanrodrigo]

dis good

[orndorffgrant]

CI fails unrelated