[SPARK-11447][SQL] change NullType to StringType during binaryComparison...

[SPARK-11447][SQL] change NullType to StringType during binaryComparison between NullType and StringType

During executing PromoteStrings rule, if one side of binaryComparison is StringType and the other side is not StringType, the current code will promote(cast) the StringType to DoubleType, and if the StringType doesn't contain the numbers, it will get null value. So if it is doing <=> (NULL-safe equal) with Null, it will not filter anything, caused the problem reported by this jira.

I proposal to the changes through this PR, can you review my code changes ?

This problem only happen for <=>, other operators works fine.

scala> val filteredDF = df.filter(df("column") > (new Column(Literal(null))))
filteredDF: org.apache.spark.sql.DataFrame = [column: string]

scala> filteredDF.show
+------+
|column|
+------+
+------+

scala> val filteredDF = df.filter(df("column") === (new Column(Literal(null))))
filteredDF: org.apache.spark.sql.DataFrame = [column: string]

scala> filteredDF.show
+------+
|column|
+------+
+------+

scala> df.registerTempTable("DF")

scala> sqlContext.sql("select * from DF where 'column' = NULL")
res27: org.apache.spark.sql.DataFrame = [column: string]

scala> res27.show
+------+
|column|
+------+
+------+

Author: Kevin Yu <qyu@us.ibm.com>

Closes #9720 from kevinyu98/working_on_spark-11447.