## What changes were proposed in this pull request?
One powerful feature of `Dataset` is, we can easily map SQL rows to Scala/Java objects and do runtime null check automatically.
For example, let's say we have a parquet file with schema `<a: int, b: string>`, and we have a `case class Data(a: Int, b: String)`. Users can easily read this parquet file into `Data` objects, and Spark will throw NPE if column `a` has null values.
However the null checking is left behind for top-level primitive values. For example, let's say we have a parquet file with schema `<a: Int>`, and we read it into Scala `Int`. If column `a` has null values, we will get some weird results.
```
scala> val ds = spark.read.parquet(...).as[Int]
scala> ds.show()
+----+
|v |
+----+
|null|
|1 |
+----+
scala> ds.collect
res0: Array[Long] = Array(0, 1)
scala> ds.map(_ * 2).show
+-----+
|value|
+-----+
|-2 |
|2 |
+-----+
```
This is because internally Spark use some special default values for primitive types, but never expect users to see/operate these default value directly.
This PR adds null check for top-level primitive values
## How was this patch tested?
new test
Author: Wenchen Fan <wenchen@databricks.com>
Closes#19707 from cloud-fan/bug.
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