[SPARK-19980][SQL] Add NULL checks in Bean serializer

## What changes were proposed in this pull request?
A Bean serializer in `ExpressionEncoder`  could change values when Beans having NULL. A concrete example is as follows;
```
scala> :paste
class Outer extends Serializable {
  private var cls: Inner = _
  def setCls(c: Inner): Unit = cls = c
  def getCls(): Inner = cls
}

class Inner extends Serializable {
  private var str: String = _
  def setStr(s: String): Unit = str = str
  def getStr(): String = str
}

scala> Seq("""{"cls":null}""", """{"cls": {"str":null}}""").toDF().write.text("data")
scala> val encoder = Encoders.bean(classOf[Outer])
scala> val schema = encoder.schema
scala> val df = spark.read.schema(schema).json("data").as[Outer](encoder)
scala> df.show
+------+
|   cls|
+------+
|[null]|
|  null|
+------+

scala> df.map(x => x)(encoder).show()
+------+
|   cls|
+------+
|[null]|
|[null]|     // <-- Value changed
+------+
```

This is because the Bean serializer does not have the NULL-check expressions that the serializer of Scala's product types has. Actually, this value change does not happen in Scala's product types;

```
scala> :paste
case class Outer(cls: Inner)
case class Inner(str: String)

scala> val encoder = Encoders.product[Outer]
scala> val schema = encoder.schema
scala> val df = spark.read.schema(schema).json("data").as[Outer](encoder)
scala> df.show
+------+
|   cls|
+------+
|[null]|
|  null|
+------+

scala> df.map(x => x)(encoder).show()
+------+
|   cls|
+------+
|[null]|
|  null|
+------+
```

This pr added the NULL-check expressions in Bean serializer along with the serializer of Scala's product types.

## How was this patch tested?
Added tests in `JavaDatasetSuite`.

Author: Takeshi Yamamuro <yamamuro@apache.org>

Closes #17347 from maropu/SPARK-19980.