[core] Ignore blank values in FieldListaggAgg operator.

[PyRSA]

Purpose

[Fix] Ignore blank input values when using field.{fieldName}.aggregate-function=listagg

Expected behavior

Case	Input A	Input B	Current Output	Expected Output
listagg	'user1,user2,user3'	'' (empty string)	'user1,user2,user3,' ❌	'user1,user2,user3'
listagg	'user1,user2,user3'	' ' (space)	'user1,user2,user3, ' ❌	'user1,user2,user3'
listagg	'user1,user2,user3'	'\t' (tab)	'user1,user2,user3,\t' ❌	'user1,user2,user3'
listagg	'user1,user2,user3'	'\n' (newline)	'user1,user2,user3,\n' ❌	'user1,user2,user3'
listagg	'user1,user2,user3'	'\r' (carriage return)	'user1,user2,user3,\r' ❌	'user1,user2,user3'
listagg	' ' (whitespace only)	'user1,user2,user3'	' ,user1,user2,user3' ❌	'user1,user2,user3'

Blank values are determined using org.apache.paimon.utils.StringUtils.StringUtils.isBlank(...).

Tests

• org.apache.paimon.mergetree.compact.aggregate.FieldAggregatorTest#testFieldListAggWithDefaultDelimiterIgnoringBlankValues
• org.apache.paimon.mergetree.compact.aggregate.FieldAggregatorTest#testFieldListAggWithDefaultDelimiterAndDistinctIgnoringBlankValues
• org.apache.paimon.mergetree.compact.aggregate.FieldAggregatorTest#testFieldListAggFirstNonBlankValueWithoutLeadingDelimiter

[JingsongLi]

This changes the non-distinct LISTAGG behavior when the accumulator is null. For the first non-blank value, mergeFieldSD is initialized as BinaryString.EMPTY_UTF8, and the final path returns concat(EMPTY, delimiter, input), so the result starts with the delimiter (for example ",first line"). This breaks the existing Flink ListAggAggregationITCase. Could we keep the old accumulator == null handling for non-blank inputs, and add a test for agg(null, BinaryString.fromString("first line"))?

[PyRSA]

This changes the non-distinct LISTAGG behavior when the accumulator is null. For the first non-blank value, mergeFieldSD is initialized as BinaryString.EMPTY_UTF8, and the final path returns concat(EMPTY, delimiter, input), so the result starts with the delimiter (for example ",first line"). This breaks the existing Flink ListAggAggregationITCase. Could we keep the old accumulator == null handling for non-blank inputs, and add a test for agg(null, BinaryString.fromString("first line"))?

Thanks for pointing this out! I've adjusted it accordingly.

[JingsongLi]

+1