[SPARK-16663][SQL] desc table should be consistent between data source and hive serde tables

## What changes were proposed in this pull request?

Currently there are 2 inconsistence:

1. for data source table, we only print partition names, for hive table, we also print partition schema. After this PR, we will always print schema
2. if column doesn't have comment, data source table will print empty string, hive table will print null. After this PR, we will always print null

## How was this patch tested?

new test in `HiveDDLSuite`

Author: Wenchen Fan <wenchen@databricks.com>

Closes #14302 from cloud-fan/minor3.