[SPARK-17916][SQL] Fix new behavior when quote is set and fix old behavior when quote is unset

[koertkuipers]

What changes were proposed in this pull request?

1. Set nullValue to quoted empty string respecting quote value
2. Fall back to old behavior of unquoted null if quote is not set

How was this patch tested?

Two new tests that will fail without these fixes

Please review http://spark.apache.org/contributing.html before opening a pull request.

[SparkQA]

Test build #95581 has finished for PR 22312 at commit ad3a11d.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #95582 has finished for PR 22312 at commit 5772404.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

#22234 was already open. Wouldn't it be able to workaround if it's configurable?