Outbox: Account for invalid json

[obenland]

Not quite sure this is the best way to go. I'd be fine with restoring init_from_json() to working with an empty array, too.

See p1738085487855409/1738079794.569609-slack-C04TJ8P900J

PHP Fatal error:  Uncaught Error: Call to undefined method WP_Error::set__id() in /wp-content/plugins/activitypub/includes/model/class-follower.php:339
Stack trace:
#0 /wp-content/plugins/activitypub/includes/collection/class-followers.php(202): Activitypub\Model\Follower::init_from_cpt(Object(WP_Post))

Proposed changes:

• Adds WP Error checks to all calls of init_from_json().
• Adds test to cover the possibility of invalid json.

Other information:

• Have you written new tests for your changes, if applicable?

[jeherve]

That fixes it, thank you!