IoTDB doesn't distinguish between zero and null values in sum aggregation

In IoTDB, the aggregate result of sum is like:


You can see that both a 0 value and a null value result in 0, without distinction.

But in **MySQL**, the aggregate result of sum is like:

I wonder if IoTDB can similarly distinguish between zero values and null values in the aggregate result of sum

Start by reproducing the sum aggregation behavior described in the issue and compare results for zero and null values with MySQL's behavior. Trace IoTDB's sum aggregation implementation and its existing tests; done means zero and null produce distinguishable aggregate results consistent with the intended semantics.

Written by the indexing model from the issue text.