fix: send null for empty edited_at in mastodon api

What does this PR do? (Please give us a brief description of what this PR does.)

While working on getting Akkoma-FE to be compatible with Sharkey, I noticed that edited_at field is not sent for statuses that weren't edited, which breaks Akkoma-FE (as it depended on that field to exist) and possibly other applications depending on Mastodon API.

Contribution Guidelines By submitting this merge request, you agree to follow our Contribution Guidelines

• I agree to follow this project's Contribution Guidelines
• I have made sure to test this pull request