[SPARK-17427][SQL] function SIZE should return -1 when parameter is null

## What changes were proposed in this pull request?

`select size(null)` returns -1 in Hive. In order to be compatible, we should return `-1`.

## How was this patch tested?

unit test in `CollectionFunctionsSuite` and `DataFrameFunctionsSuite`.

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #14991 from adrian-wang/size.