## 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.
556a3b7d07