RDS: Fix keep_days zero values

[artem-lifshits]

Summary of the Pull Request

Fix resource break when keep_days is set to 0.

PR Checklist

• Refers to: RDSv3: Error "missing input for argument [KeepDays]" when keep_days is 0 #2262
• Tests passed.
• Release notes added.

Acceptance Steps Performed

=== RUN   TestAccRdsPostgre13V3ParamsBasic
--- PASS: TestAccRdsPostgre13V3ParamsBasic (457.13s)
=== RUN   TestAccRdsInstanceV3Basic
--- PASS: TestAccRdsInstanceV3Basic (836.01s)
PASS

Process finished with the exit code 0

[otc-zuul]

Build succeeded.
https://zuul.otc-service.com/t/eco/buildset/8dee23b245484511b22c43dff116556b

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✔️ golang-make-test SUCCESS in 4m 26s
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✔️ goreleaser-build SUCCESS in 7m 05s