[SPARK-17328][SQL] Fix NPE with EXPLAIN DESCRIBE TABLE

## What changes were proposed in this pull request?

This PR fixes the following NPE scenario in two ways.

**Reported Error Scenario**
```scala
scala> sql("EXPLAIN DESCRIBE TABLE x").show(truncate = false)
INFO SparkSqlParser: Parsing command: EXPLAIN DESCRIBE TABLE x
java.lang.NullPointerException
```

- **DESCRIBE**: Extend `DESCRIBE` syntax to accept `TABLE`.
- **EXPLAIN**: Prevent NPE in case of the parsing failure of target statement, e.g., `EXPLAIN DESCRIBE TABLES x`.

## How was this patch tested?

Pass the Jenkins test with a new test case.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #15357 from dongjoon-hyun/SPARK-17328.