[SPARK-15875] Try to use Seq.isEmpty and Seq.nonEmpty instead of Seq.length == 0 and Seq.length > 0

## What changes were proposed in this pull request?

In scala, immutable.List.length is an expensive operation so we should
avoid using Seq.length == 0 or Seq.lenth > 0, and use Seq.isEmpty and Seq.nonEmpty instead.

## How was this patch tested?
existing tests

Author: wangyang <wangyang@haizhi.com>

Closes #13601 from yangw1234/isEmpty.