…32134)

In Node.js, if you assign `undefined` to an environment variable, that variable ends up having the string `"undefined"`.

If we are using IAM user credentials, `AWS_SESSION_TOKEN` should not be set, but because we were not handling this edge case, it was getting assigned an invalid value:

```
Welcome to Node.js v22.9.0.
Type ".help" for more information.
> process.env.AWS_SESSION_TOKEN || process.env.AMAZON_SESSION_TOKEN
undefined
> process.env.AWS_SESSION_TOKEN = process.env.AWS_SESSION_TOKEN || process.env.AMAZON_SESSION_TOKEN
undefined
> process.env.AWS_SESSION_TOKEN
'undefined'
```

Closes #32120.

### Checklist
- [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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We just didn't consider the shared credentials file as returned by the SDK when loading configuration. 

Closes #32130.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*