Set response values to zero if None for /_synapse/admin/v1/federation/destinations

[Half-Shot]

The docs say these values should be integers (0 if no retries have been tried), but Synapse is currently returning null instead.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Pull request includes a sign off
• Code style is correct
  (run the linters)

[DMRobertson]

Those docs are https://matrix-org.github.io/synapse/latest/usage/administration/admin_api/federation.html#list-of-destinations

SGTM. Thanks!