[SPARK-14338][SQL] Improve `SimplifyConditionals` rule to handle `null` in IF/CASEWHEN

## What changes were proposed in this pull request?

Currently, `SimplifyConditionals` handles `true` and `false` to optimize branches. This PR improves `SimplifyConditionals` to take advantage of `null` conditions for `if` and `CaseWhen` expressions, too.

**Before**
```
scala> sql("SELECT IF(null, 1, 0)").explain()
== Physical Plan ==
WholeStageCodegen
:  +- Project [if (null) 1 else 0 AS (IF(CAST(NULL AS BOOLEAN), 1, 0))#4]
:     +- INPUT
+- Scan OneRowRelation[]
scala> sql("select case when cast(null as boolean) then 1 else 2 end").explain()
== Physical Plan ==
WholeStageCodegen
:  +- Project [CASE WHEN null THEN 1 ELSE 2 END AS CASE WHEN CAST(NULL AS BOOLEAN) THEN 1 ELSE 2 END#14]
:     +- INPUT
+- Scan OneRowRelation[]
```

**After**
```
scala> sql("SELECT IF(null, 1, 0)").explain()
== Physical Plan ==
WholeStageCodegen
:  +- Project [0 AS (IF(CAST(NULL AS BOOLEAN), 1, 0))#4]
:     +- INPUT
+- Scan OneRowRelation[]
scala> sql("select case when cast(null as boolean) then 1 else 2 end").explain()
== Physical Plan ==
WholeStageCodegen
:  +- Project [2 AS CASE WHEN CAST(NULL AS BOOLEAN) THEN 1 ELSE 2 END#4]
:     +- INPUT
+- Scan OneRowRelation[]
```

**Hive**
```
hive> select if(null,1,2);
OK
2
hive> select case when cast(null as boolean) then 1 else 2 end;
OK
2
```

## How was this patch tested?

Pass the Jenkins tests (including new extended test cases).

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12122 from dongjoon-hyun/SPARK-14338.