[SPARK-19271][SQL] Change non-cbo estimation of aggregate

## What changes were proposed in this pull request?

Change non-cbo estimation behavior of aggregate:
- If groupExpression is empty, we can know row count (=1) and the corresponding size;
- otherwise, estimation falls back to UnaryNode's computeStats method, which should not propagate rowCount and attributeStats in Statistics because they are not estimated in that method.

## How was this patch tested?

Added test case

Author: wangzhenhua <wangzhenhua@huawei.com>

Closes #16631 from wzhfy/aggNoCbo.