[SPARK-18269][SQL] CSV datasource should read null properly when schema is lager than parsed tokens

## What changes were proposed in this pull request?

Currently, there are the three cases when reading CSV by datasource when it is `PERMISSIVE` parse mode.

- schema == parsed tokens (from each line)
  No problem to cast the value in the tokens to the field in the schema as they are equal.

- schema < parsed tokens (from each line)
  It slices the tokens into the number of fields in schema.

- schema > parsed tokens (from each line)
  It appends `null` into parsed tokens so that safely values can be casted with the schema.

However, when `null` is appended in the third case, we should take `null` into account when casting the values.

In case of `StringType`, it is fine as `UTF8String.fromString(datum)` produces `null` when the input is `null`. Therefore, this case will happen only when schema is explicitly given and schema includes data types that are not `StringType`.

The codes below:

```scala
val path = "/tmp/a"
Seq("1").toDF().write.text(path.getAbsolutePath)
val schema = StructType(
  StructField("a", IntegerType, true) ::
  StructField("b", IntegerType, true) :: Nil)
spark.read.schema(schema).option("header", "false").csv(path).show()
```

prints

**Before**

```
java.lang.NumberFormatException: null
at java.lang.Integer.parseInt(Integer.java:542)
at java.lang.Integer.parseInt(Integer.java:615)
at scala.collection.immutable.StringLike$class.toInt(StringLike.scala:272)
at scala.collection.immutable.StringOps.toInt(StringOps.scala:29)
at org.apache.spark.sql.execution.datasources.csv.CSVTypeCast$.castTo(CSVInferSchema.scala:24)
```

**After**

```
+---+----+
|  a|   b|
+---+----+
|  1|null|
+---+----+
```

## How was this patch tested?

Unit test in `CSVSuite.scala` and `CSVTypeCastSuite.scala`

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #15767 from HyukjinKwon/SPARK-18269.