fix(rds-signer): don't overwrite default config with undefined values

[batzz-00]

Issue

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Description

What does this implement/fix? Explain your changes.
The documentation for the credentials option would lead you to think an explicit undefined or omitting the option would use the default credential provider, the current implementation only works for the latter scenario. Explicitly passing undefined uses the default provider in v2.

In the scenario that a user provides credentials: undefined in the config object, because it is spread at the end of the object, the defaulted properties are overwritten

Testing

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Additional context

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Checklist

• If you wrote E2E tests, are they resilient to concurrent I/O?
• If adding new public functions, did you add the @public tag and enable doc generation on the package?

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