Fix NullPointerException in Kinesis binder CloudWatch client creation

[kobeomseok95]

📢 Type of change

• Bugfix
• New feature
• Enhancement
• Refactoring

📜 Description

Fixes a NullPointerException in KinesisBinderConfiguration.cloudWatch() that occurs when spring.cloud.stream.kinesis.binder.kpl-kcl-enabled=true is set, the application has at least one input binding, and no AwsClientCustomizer<CloudWatchAsyncClientBuilder> bean is registered.

The method passed the optional customizer as Stream.of(configurer.getIfAvailable()), which produces a stream containing a single null element when the bean is absent, causing the NPE inside AwsClientBuilderConfigurer.configureAsyncClient(...). This change uses configurer.stream() instead, aligning cloudWatch() with the sibling client bean methods (amazonKinesis(), dynamoDB(), dynamoDBStreams()), and removes the now-unused java.util.stream.Stream import.

💡 Motivation and Context

Without this fix, enabling the KPL/KCL mode of the Kinesis binder requires registering a no-op AwsClientCustomizer<CloudWatchAsyncClientBuilder> bean as a workaround.

Fixes #1651

💚 How did you test it?

• Added KinesisBinderConfigurationTests with two tests:
  • cloudWatchClientIsCreatedWhenNoCustomizerBeanIsRegistered — reproduces the reported NPE before the fix and passes after it
  • cloudWatchClientAppliesRegisteredCustomizer — verifies a registered customizer is still applied after the change
• Ran the full spring-cloud-aws-kinesis-stream-binder test suite (LocalStack/Testcontainers based) — all 25 tests pass

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• I updated reference documentation to reflect the change (not needed — internal bugfix, no API or documented behavior change)
• All tests passing
• No breaking changes

🔮 Next steps

The bug is present in the released 4.0.x line (verified on 4.0.2), so this may be worth backporting to 4.0.x.