[SPARK-18753][SQL] Keep pushed-down null literal as a filter in Spark-side...

[SPARK-18753][SQL] Keep pushed-down null literal as a filter in Spark-side post-filter for FileFormat datasources

## What changes were proposed in this pull request?

Currently, `FileSourceStrategy` does not handle the case when the pushed-down filter is `Literal(null)` and removes it at the post-filter in Spark-side.

For example, the codes below:

```scala
val df = Seq(Tuple1(Some(true)), Tuple1(None), Tuple1(Some(false))).toDF()
df.filter($"_1" === "true").explain(true)
```

shows it keeps `null` properly.

```
== Parsed Logical Plan ==
'Filter ('_1 = true)
+- LocalRelation [_1#17]

== Analyzed Logical Plan ==
_1: boolean
Filter (cast(_1#17 as double) = cast(true as double))
+- LocalRelation [_1#17]

== Optimized Logical Plan ==
Filter (isnotnull(_1#17) && null)
+- LocalRelation [_1#17]

== Physical Plan ==
*Filter (isnotnull(_1#17) && null)       << Here `null` is there
+- LocalTableScan [_1#17]
```

However, when we read it back from Parquet,

```scala
val path = "/tmp/testfile"
df.write.parquet(path)
spark.read.parquet(path).filter($"_1" === "true").explain(true)
```

`null` is removed at the post-filter.

```
== Parsed Logical Plan ==
'Filter ('_1 = true)
+- Relation[_1#11] parquet

== Analyzed Logical Plan ==
_1: boolean
Filter (cast(_1#11 as double) = cast(true as double))
+- Relation[_1#11] parquet

== Optimized Logical Plan ==
Filter (isnotnull(_1#11) && null)
+- Relation[_1#11] parquet

== Physical Plan ==
*Project [_1#11]
+- *Filter isnotnull(_1#11)       << Here `null` is missing
   +- *FileScan parquet [_1#11] Batched: true, Format: ParquetFormat, Location: InMemoryFileIndex[file:/tmp/testfile], PartitionFilters: [null], PushedFilters: [IsNotNull(_1)], ReadSchema: struct<_1:boolean>
```

This PR fixes it to keep it properly. In more details,

```scala
val partitionKeyFilters =
  ExpressionSet(normalizedFilters.filter(_.references.subsetOf(partitionSet)))
```

This keeps this `null` in `partitionKeyFilters` as `Literal` always don't have `children` and `references` is being empty  which is always the subset of `partitionSet`.

And then in

```scala
val afterScanFilters = filterSet -- partitionKeyFilters
```

`null` is always removed from the post filter. So, if the referenced fields are empty, it should be applied into data columns too.

After this PR, it becomes as below:

```
== Parsed Logical Plan ==
'Filter ('_1 = true)
+- Relation[_1#276] parquet

== Analyzed Logical Plan ==
_1: boolean
Filter (cast(_1#276 as double) = cast(true as double))
+- Relation[_1#276] parquet

== Optimized Logical Plan ==
Filter (isnotnull(_1#276) && null)
+- Relation[_1#276] parquet

== Physical Plan ==
*Project [_1#276]
+- *Filter (isnotnull(_1#276) && null)
   +- *FileScan parquet [_1#276] Batched: true, Format: ParquetFormat, Location: InMemoryFileIndex[file:/private/var/folders/9j/gf_c342d7d150mwrxvkqnc180000gn/T/spark-a5d59bdb-5b..., PartitionFilters: [null], PushedFilters: [IsNotNull(_1)], ReadSchema: struct<_1:boolean>
```

## How was this patch tested?

Unit test in `FileSourceStrategySuite`

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #16184 from HyukjinKwon/SPARK-18753.

(cherry picked from commit 89ae26dc)
Signed-off-by: Cheng Lian <lian@databricks.com>