[SPARK-19709][SQL] Read empty file with CSV data source

## What changes were proposed in this pull request?

Bugfix for reading empty file with CSV data source. Instead of throwing `NoSuchElementException`, an empty data frame is returned.

## How was this patch tested?

Added new unit test in `org.apache.spark.sql.execution.datasources.csv.CSVSuite`

Author: Wojtek Szymanski <wk.szymanski@gmail.com>

Closes #17068 from wojtek-szymanski/SPARK-19709.