## What changes were proposed in this pull request?
```Scala
val df1 = Seq(("abc", 1), (null, 3)).toDF("col1", "col2")
df1.write.mode(SaveMode.Overwrite).parquet("/tmp/test1")
val df2 = spark.read.parquet("/tmp/test1")
df2.filter("col1 = 'abc' OR (col1 != 'abc' AND col2 == 3)").show()
```
Before the PR, it returns both rows. After the fix, it returns `Row ("abc", 1))`. This is to fix the bug in NULL handling in BooleanSimplification. This is a bug introduced in Spark 1.6 release.
## How was this patch tested?
Added test cases
Closes#22702 from gatorsmile/fixBooleanSimplify2.
Authored-by: gatorsmile <gatorsmile@gmail.com>
Signed-off-by: gatorsmile <gatorsmile@gmail.com>
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