[SPARK-29187][SQL] Return null from date_part() for the null field

[MaxGekk]

What changes were proposed in this pull request?

In the PR, I propose to change behavior of the date_part() function in handling null field, and make it the same as PostgreSQL has. If field parameter is null, the function should return null of the double type as PostgreSQL does:

# select date_part(null, date '2019-09-20');
 date_part 
-----------
          
(1 row)

# select pg_typeof(date_part(null, date '2019-09-20'));
    pg_typeof     
------------------
 double precision
(1 row)

Why are the changes needed?

The date_part() function was added to maintain feature parity with PostgreSQL but current behavior of the function is different in handling null as field.

Does this PR introduce any user-facing change?

Yes.

Before:

spark-sql> select date_part(null, date'2019-09-20');
Error in query: null; line 1 pos 7

After:

spark-sql> select date_part(null, date'2019-09-20');
NULL

How was this patch tested?

Add new tests to `DateFunctionsSuite for 2 cases:

• field = null, source = a date literal
• field = null, source = a date column

[adrian-wang]

DoubleType ?

[MaxGekk]

See the example in the description. PostgreSQL returns NULL of the double precision type which is equivalent of DOUBLE type in Spark SQL. Which type would you use here?

[SparkQA]

Test build #111056 has finished for PR 25865 at commit b111cf2.

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

[SparkQA]

Test build #111058 has finished for PR 25865 at commit b39adaa.

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

[dongjoon-hyun]

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

[dongjoon-hyun]

Also, thank you for review, @adrian-wang .