Bug: group by aggregation on internal table gives a wrong result

[chenzl25]

Describe the bug

dev=> explain select count(*) from __internal_v_4_hashjoinleft_1003 group by t_a;
                                QUERY PLAN
--------------------------------------------------------------------------
 BatchSimpleAgg { aggs: [sum0(count)] }
 └─BatchExchange { order: [], dist: Single }
   └─BatchSimpleAgg { aggs: [count] }
     └─BatchScan { table: __internal_v_4_hashjoinleft_1003, columns: [] }

Error message/log

No response

To Reproduce

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Expected behavior

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How did you deploy RisingWave?

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The version of RisingWave

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Additional context

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[BugenZhao]

#7413