Aggregation's output state should be NULL on initialization

[kwannoel]

Describe the bug

Aggregation executors (including approx percentile) currently only output their state in-between initial barrier and second barrier.

This means before that the materialized state is empty, rather than NULL.

It's a minor issue though. Since after the second barrier the state is normal.

Given that it's tricky to fix and not a critical issue (materialize expects barrier initially, and most executors don't expect any data in the first epoch committed to their state table), I will mark it as low priority.

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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[github-actions]

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