[FLINK-36267][table] Fix SPLIT not preserving SMP characters when delimiter is empty

[Au-Miner]

What is the purpose of the change

Fix the SQL SPLIT function so that, when called with an empty delimiter, it preserves Supplementary Multilingual Plane characters (e.g. emoji) as single elements instead of breaking surrogate pairs into invalid "?" placeholders.

Brief change log

• Change the empty-delimiter branch in SplitFunction#eval to iterate by Unicode code points (codePointAt + Character.charCount) instead of by UTF-16 code units, so surrogate-pair characters are emitted as one element.

Verifying this change

Added a new test case in CollectionFunctionsITCase#splitTestCases that asserts SPLIT('123😊笑脸', '') returns ["1", "2", "3", "😊", "笑", "脸"], covering the SMP character path across both Table API and SQL.

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: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• c784b25 Azure: SUCCESS

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