fix(cli): cli still fails for some plugins returning expiration: null (#32668)

There is one more location where we need to convert `null` into `undefined` if a plugin provider returns `expiration: null`. The SDKv3 code base also expects this value to be `undefined | Date`, just like our own code does; when the plugin returns `null` we pass the value over to the SDKv3 which then does a `getTime()` on the value, and then it crashes.

Issue this fix as a patch, so that we can unblock users of a commonly used plugin at Amazon.

This is a PR targeting `v2-release`, picking exactly one line from #32111. After merging this we can patch-release.


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

Author: rix0rrr

packages/aws-cdk/lib/api/aws-auth/credential-plugins.ts

@@ -140,7 +140,7 @@ function v3ProviderFromV2Credentials(x: SDKv2CompatibleCredentials): AwsCredenti
       accessKeyId: x.accessKeyId,
       secretAccessKey: x.secretAccessKey,
       sessionToken: x.sessionToken,
-      expiration: x.expireTime,
+      expiration: x.expireTime ?? undefined,
     };
   };
 }