postgres: Allow ignore_databases config option to be empty

[xcoderzach]

What does this PR do?

This fixes a bug where setting ignore_databases: [] in the config would generate a broken query.

Motivation

A client is using databases that we typically ignore, and would like to monitor them.

Additional Notes

TODO: I'm not sure where the best place to add a test for this is.

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] PR title must be written as a CHANGELOG entry (see why)
• [ ] Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• [ ] PR must have changelog/ and integration/ labels attached

[codecov[bot]]

Codecov Report

Merging #12021 (1361e8d) into master (caab6f6) will increase coverage by 0.01%. Report is 3354 commits behind head on master. The diff coverage is 100.00%.

Flag	Coverage Δ
postgres	92.09% <100.00%> (+0.42%)	:arrow_up:

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[FlorentClarret]

Closing this PR since it's still a draft, feel free to re-open it if needed 🙂