[SPARK-29162][SQL] Simplify NOT(IsNull(x)) and NOT(IsNotNull(x))

### What changes were proposed in this pull request?
Rewrite
```
NOT isnull(x)     -> isnotnull(x)
NOT isnotnull(x)  -> isnull(x)
```

### Why are the changes needed?
Make LogicalPlan more readable and  useful for query canonicalization. Make same condition equal when judge query canonicalization equal

### Does this PR introduce any user-facing change?

NO

### How was this patch tested?

Newly added UTs.

Closes apache#25878 from AngersZhuuuu/SPARK-29162.

Authored-by: angerszhu <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/expressions.scala

@@ -373,6 +373,9 @@ object BooleanSimplification extends Rule[LogicalPlan] with PredicateHelper {
       case Not(a And b) => Or(Not(a), Not(b))
 
       case Not(Not(e)) => e
+
+      case Not(IsNull(e)) => IsNotNull(e)
+      case Not(IsNotNull(e)) => IsNull(e)
     }
   }
 }

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/BooleanSimplificationSuite.scala

@@ -239,6 +239,11 @@ class BooleanSimplificationSuite extends PlanTest with ExpressionEvalHelper with
     checkCondition(!'f || 'e, testRelationWithData.where(!'f || 'e).analyze)
   }
 
+  test("simplify NOT(IsNull(x)) and NOT(IsNotNull(x))") {
+    checkCondition(Not(IsNotNull('b)), IsNull('b))
+    checkCondition(Not(IsNull('b)), IsNotNull('b))
+  }
+
   protected def assertEquivalent(e1: Expression, e2: Expression): Unit = {
     val correctAnswer = Project(Alias(e2, "out")() :: Nil, OneRowRelation()).analyze
     val actual = Optimize.execute(Project(Alias(e1, "out")() :: Nil, OneRowRelation()).analyze)