[FLINK-39607][s3] Add missing Nullable annotation and precondition for config

[Samrat002]

What is the purpose of the change

This pull request cleans up null handling in the flink-s3-fs-native module by adding proper preconditions for non-nullable constructor parameters, annotating nullable fields explicitly, and replacing ad-hoc null && isEmpty() checks with StringUtils.isNullOrWhitespaceOnly.

Brief change log

• Replace Objects.requireNonNull with Preconditions.checkNotNull (Flink convention) for all required constructor parameters in S3ClientProvider, NativeS3FileSystem, and NativeS3OutputStream
• Remove @nullable from encryptionConfig constructor parameters in S3ClientProvider and NativeS3OutputStream; default the Builder field to S3EncryptionConfig.none() so the contract is non-null end-to-end
• Remove dead null guards on s3Client and transferManager in S3ClientProvider.closeAsync(), and on clientProvider in NativeS3FileSystem.closeAsync().
• Replace manual != null && !isEmpty() string checks in S3ClientProvider.Builder with StringUtils.isNullOrWhitespaceOnly for assumeRoleArn, assumeRoleExternalId, explicitRegion, and credentialsProviderClasses
• Fix S3ClientProvider.Builder.assumeRoleSessionName() to drop its silent null-guard (the config option has a default value, null was never a valid input)
• Fix the async S3AsyncClient builder to apply endpointOverride conditionally, consistent with the sync S3Client builder
• Add @nullable annotation to NativeS3FileSystemFactory.flinkConfig field to make the deferred-initialization contract explicit

Verifying this change

Existing tests cover the affected code paths:

• S3ClientProviderTest
• NativeS3FileSystemFactoryTest
• S3EncryptionConfigTest

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: no
• The S3 file system connector: yes

Documentation

• Does this pull request introduce a new feature? no

---

Was generative AI tooling used to co-author this PR?

• No

[flinkbot]

CI report:

• 14503c4 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[Samrat002]

@gaborgsomogyi PTAL