[SPARK-19543] from_json fails when the input row is empty

## What changes were proposed in this pull request?

Using from_json on a column with an empty string results in: java.util.NoSuchElementException: head of empty list.

This is because `parser.parse(input)` may return `Nil` when `input.trim.isEmpty`

## How was this patch tested?

Regression test in `JsonExpressionsSuite`

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #16881 from brkyvz/json-fix.