Fix aggregation NPE when child input finishes early

[Caideyipi]

Description

• Skip missing child TsBlocks when merging partial aggregation results.
• Avoid advancing null child input blocks after a child operator has finished.
• Add a regression test for group-by intermediate aggregation when one child has fewer windows than another.

Tests

• mvn -nsu -pl iotdb-core/datanode -am -Dtest=AggregationOperatorTest#testGroupByIntermediateResultWithFinishedChild -DfailIfNoTests=false "-Dsurefire.failIfNoSpecifiedTests=false" test

[PDGGK]

Thanks for fixing this — the null-guards read well. Two small questions to make sure I follow the intended behavior:

1. In the isInputFinal branch, when the input block is null the method returns without calling accumulator.setFinal(...). Is the intent that the accumulator keeps its prior state (no final result set for this call) and a later non-null block sets it? Just confirming there's no path where the final result ends up unset because the producing child finished with a null block.
2. In the intermediate branch, hasMissingInput breaks on the first null and skips the whole group. I believe that's safe because a given InputLocation[] group's columns all come from the same child's TsBlock (same tsBlockIndex), so if one is null they all are — is that assumption always true, or could a group ever span multiple children where one finished early?

Neither is blocking — just making sure I understand the model. Thanks!