[SPARK-20680][SQL][FOLLOW-UP] Revert NullType.simpleString from 'unknown' to 'null'

[HyukjinKwon]

What changes were proposed in this pull request?

This PR proposes to partially reverts the simple string in NullType at #28833: NullType.simpleString back from unknown to null.

Why are the changes needed?

• Technically speaking, it's orthogonal with the issue itself, SPARK-20680.
• It needs some more discussion, see [SPARK-20680][SQL] Spark-sql do not support for creating table with void column datatype #28833 (comment)

Does this PR introduce any user-facing change?

It reverts back the user-facing changes at #28833.
The simple string of NullType is back to null.

How was this patch tested?

I just logically reverted. Jenkins should test it out.

[dongjoon-hyun]

Retest this please.

[SparkQA]

Test build #125432 has finished for PR 29041 at commit 474cc70.

• This patch fails due to an unknown error code, -9.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

retest this please

[SparkQA]

Test build #125442 has finished for PR 29041 at commit 474cc70.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

Retest this please.

[SparkQA]

Test build #125503 has finished for PR 29041 at commit 474cc70.

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

[dongjoon-hyun]

+1, LGTM. Merged to master. Thank you, @HyukjinKwon .

[HyukjinKwon]

Thank you @dongjoon-hyun