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

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.

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,
     };
   };
 }