planner: mpp-agg output wrong result for empty set in the mock handler of unistore

[AilinKid]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

say we got a count[MPP] for mpp handler in unistore, and we split it as two partial aggregation.

sum(#1) as #2 [MPP]
   +---count(source) as #1  [MPP]
             +--- source (let it be empty set)  [MPP]

currently mpp handler will output nil (chunk size as 0 for #1) facing another empty set of source
currently mpp handler will output nil (chunk size as 0 for #2) facing another empty set

2. What did you expect to see? (Required)

we get 0 of #1 even for an empty set of source
we get 0 of #2 because it has 1 row as 0.

3. What did you see instead (Required)

return empty set

4. What is your TiDB version? (Required)

master