fix(rds): grantConnect fails to deploy when no user is specified for instances with secret credentials

[lpizzinidev]

Calling grantConnect on instances with secret credentials not specifying the user was causing the deploy to fail because the secret was not resolved.
This fix solves the bug.

Closes #26603.

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[rix0rrr]

Looks good, and I'm ready to ship it! However, I'm noticing your PR titles have not followed the contributing guide best practices at least a couple of times, so I'm going to set this to "Request changes" to make sure that you see this comment 😇

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Please make sure that your PR title confirms to the conventional commit standard (fix, feat, chore) and that it is written in a style that will reflect correctly in the change log. In particular, if this is a bug fix, please describe the bug that was fixed, not the change you made. (See Contributing Guide, Pull Requests)

Specifically, this part:

Titles for feat and fix PRs end up in the change log. Think about what makes most sense for users reading the changelog while writing them.

• feat: describe the feature (not the action of creating the commit or PR, for example, avoid words like "added" or "changed")
• fix: describe the bug (not the solution)

[lpizzinidev]

@rix0rrr Thanks for the feedback 👍

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Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

[aws-cdk-automation]

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• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 08c5a35
• Result: SUCCEEDED
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[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).