[core] Adjust distinct option behavior in FieldListaggAgg operator

[759434091]

[core] Adjust distinct option in FieldListaggAgg operator

Purpose

1. [Fix] Fix field.{fieldName}.aggregate-function=listagg behavior when handling empty strings
2. [Feature] Adjust field.{fieldName}.distict behavior when aggregating strings

Expected behavior:

case	Input A	Input B	Current Output	Expected Output
listagg	'user1,user2,user3'	'' (empty string)	'user1,user2,user3,' ❌	'user1,user2,user3'
listagg & distinct #6566	'user1,user2,user3'	'user1,user4'	'user1,user2,user3,user1,user4' ❌	'user1,user2,user3,user4'
listagg	'' (empty string)	'user1,user2,user3'	',user1,user2,user3' ❌	'user1,user2,user3'

Tests

• org.apache.paimon.mergetree.compact.aggregate.FieldAggregatorTest#testFieldListAggWithDefaultDelimiterAndDistinct
• org.apache.paimon.mergetree.compact.aggregate.FieldAggregatorTest#testFieldListAggWithDefaultDelimiterAndDistinctWithMultiUser
• org.apache.paimon.mergetree.compact.aggregate.FieldAggregatorTest#testFieldListAggWithDefaultDelimiterAndDistinctWithEmptyLeftUser
• org.apache.paimon.mergetree.compact.aggregate.FieldAggregatorTest#testFieldListAggWithDefaultDelimiterAndDistinctWithMultiKvString
• org.apache.paimon.mergetree.compact.aggregate.FieldAggregatorTest#testFieldListAggWithDefaultDelimiterAndDistinctWithMultiDuplicatedKvString

API and Format

No Changes.

Documentation

docs/content/primary-key-table/merge-engine/aggregation.md