Check Postgres backup is available before wiping and restoring

[tumbledwyer]

Description

If a staging server is upgraded to v1.9, it adds postgres.
But since Prod will take a while to get upgraded, there will be a period where Staging has Postgres, while Prod does not.
So when Prod backs up to Staging, there will be no Postgres backup.

Currently, when staging attempts to restore, it wipes all dbs first and then attempts to restore.
The postgres restore will fail and all data will remain wiped.

This PR first check if a postgres backup exists before attempting to wipe or restore.

Fixes: opencrvs/opencrvs-core#10682

Link this pull request to the GitHub issue (and optionally name the branch ocrvs-<issue #>)

Checklist

• I have linked the correct Github issue under "Development"
• I have tested the changes locally, and written appropriate tests
• I have tested beyond the happy path (e.g. edge cases, failure paths)
• I have updated the changelog with this change (if applicable)
• I have updated the GitHub issue status accordingly

[github-actions]

Oops! Looks like you forgot to update the changelog. When updating CHANGELOG.md, please consider the following:

• Changelog is read by country implementors who might not always be familiar with all technical details of OpenCRVS. Keep language high-level, user friendly and avoid technical references to internals.
• Answer "What's new?", "Why was the change made?" and "Why should I care?" for each change.
• If it's a breaking change, include a migration guide answering "What do I need to do to upgrade?".

[cibelius]

Nice 👍