Fix healthcheck for empty pools

[sunil-2000]

Summary

• Fixes ELB HTTP healthcheck fails with passthrough auth #983
• Treat an empty pool set as healthy for the HTTP healthcheck.
• Preserve the existing behavior where PgDog reports down when all existing pools are unhealthy.

Tests

• cargo fmt
• cargo nextest run --profile dev no_pools_is_healthy

[CLAassistant]

All committers have signed the CLA.

[levkk]

Easy 🚀

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/healthcheck.rs	92.30%	1 Missing ⚠️

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