[SPARK-18260] Make from_json null safe

## What changes were proposed in this pull request?

`from_json` is currently not safe against `null` rows. This PR adds a fix and a regression test for it.

## How was this patch tested?

Regression test

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #15771 from brkyvz/json_fix.

(cherry picked from commit 6e270181)
Signed-off-by: Reynold Xin <rxin@databricks.com>