fix: clarify behavior of agg funcs regarding to nulls

[stdrc]

Fix https://github.com/risingwavelabs/risingwave-docs-legacy/issues/2757

[WanYixian]

LGTM, wondering if it's safe to replace "no non-null values" with "no valid values" in this context 🤔

[stdrc]

wondering if it's safe to replace "no non-null values" with "no valid values" in this context 🤔

Not safe. null is a term in SQL, "no non-null values" is more accurate and professional than "no valid values".

[WanYixian]

wondering if it's safe to replace "no non-null values" with "no valid values" in this context 🤔

Not safe. null is a term in SQL, "no non-null values" is more accurate and professional than "no valid values".

Yeah non-null is commonly seen, but "no non-null" seems not that immediately clear.
Since no other precise alternative, let's stick with "no non-null".