[SPARK-19218][SQL] Fix SET command to show a result correctly and in a sorted order

## What changes were proposed in this pull request?

This PR aims to fix the following two things.

1. `sql("SET -v").collect()` or `sql("SET -v").show()` raises the following exceptions for String configuration with default value, `null`. For the test, please see [Jenkins result](https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/71539/testReport/) and https://github.com/apache/spark/commit/60953bf1f1ba144e709fdae3903a390ff9479fd0 in #16624 .

```
sbt.ForkMain$ForkError: java.lang.RuntimeException: Error while decoding: java.lang.NullPointerException
createexternalrow(input[0, string, false].toString, input[1, string, false].toString, input[2, string, false].toString, StructField(key,StringType,false), StructField(value,StringType,false), StructField(meaning,StringType,false))
:- input[0, string, false].toString
:  +- input[0, string, false]
:- input[1, string, false].toString
:  +- input[1, string, false]
+- input[2, string, false].toString
   +- input[2, string, false]
```

2. Currently, `SET` and `SET -v` commands show unsorted result.
    We had better show a sorted result for UX. Also, this is compatible with Hive.

**BEFORE**
```
scala> sql("set").show(false)
...
|spark.driver.host              |10.22.16.140                                                                                                                                 |
|spark.driver.port              |63893                                                                                                                                        |
|spark.repl.class.uri           |spark://10.22.16.140:63893/classes                                                                                                           |
...
|spark.app.name                 |Spark shell                                                                                                                                  |
|spark.driver.memory            |4G                                                                                                                                           |
|spark.executor.id              |driver                                                                                                                                       |
|spark.submit.deployMode        |client                                                                                                                                       |
|spark.master                   |local[*]                                                                                                                                     |
|spark.home                     |/Users/dhyun/spark                                                                                                                           |
|spark.sql.catalogImplementation|hive                                                                                                                                         |
|spark.app.id                   |local-1484333618945                                                                                                                          |
```

**AFTER**

```
scala> sql("set").show(false)
...
|spark.app.id                   |local-1484333925649                                                                                                                          |
|spark.app.name                 |Spark shell                                                                                                                                  |
|spark.driver.host              |10.22.16.140                                                                                                                                 |
|spark.driver.memory            |4G                                                                                                                                           |
|spark.driver.port              |64994                                                                                                                                        |
|spark.executor.id              |driver                                                                                                                                       |
|spark.jars                     |                                                                                                                                             |
|spark.master                   |local[*]                                                                                                                                     |
|spark.repl.class.uri           |spark://10.22.16.140:64994/classes                                                                                                           |
|spark.sql.catalogImplementation|hive                                                                                                                                         |
|spark.submit.deployMode        |client                                                                                                                                       |
```

## How was this patch tested?

Jenkins with a new test case.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #16579 from dongjoon-hyun/SPARK-19218.