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Jiayi Liao avatarJiayi Liao

[FLINK-14296][sql-parser] Improve handling of parameters nullabillity in parser module

1. Add @Nullable annotation to nullable fields
2. Use Optional instead of nullable as return value
3. Add requrieNonNull check in constructor for non-null fields
4. List fields do not need to check against null

This closes #9843