[SPARK-18852][SS] StreamingQuery.lastProgress should be null when recentProgress is empty

## What changes were proposed in this pull request?

Right now `StreamingQuery.lastProgress` throws NoSuchElementException and it's hard to be used in Python since Python user will just see Py4jError.

This PR just makes it return null instead.

## How was this patch tested?

`test("lastProgress should be null when recentProgress is empty")`

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #16273 from zsxwing/SPARK-18852.