[SPARK-15977][SQL] Fix TRUNCATE TABLE for Spark specific datasource tables

## What changes were proposed in this pull request?
`TRUNCATE TABLE` is currently broken for Spark specific datasource tables (json, csv, ...). This PR correctly sets the location for these datasources which allows them to be truncated.

## How was this patch tested?
Extended the datasources `TRUNCATE TABLE` tests in `DDLSuite`.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #13697 from hvanhovell/SPARK-15977.