Empty tables use group functions to prompt empty set

Empty tables use group functions to prompt empty set

## Expected Behavior
count(*) => 0

## Current Behavior
empty set

## case
```yaml
- id: 3
desc: "表是空的"
tags: ["TODO","@chengjing,bug,"]
inputs:
-
columns : ["id bigint","c1 string","c2 int","c7 timestamp"]
indexs: ["index1:c1:c7"]
sql: select count(c2) as v1,max(c2) as v2,min(c2) as v3,avg(c2) as v4,sum(c2) as v5 from {0};
expect:
order: c1
columns: ["v1 int","v2 int","v3 int","v4 double","v5 bigint"]
rows:
- [0,0,0,0,0]
```

The issue includes a test case in YAML showing that aggregate functions (count, max, min, avg, sum) on an empty table return 'empty set' instead of zeros. Look for the SQL execution engine code handling aggregate functions and empty result sets. Run the provided test to reproduce the issue, then examine how aggregate functions compute results when no rows are present.

Scritto dal modello di indicizzazione a partire dal testo della issue.