[SPARK-14757] [SQL] Fix nullability bug in EqualNullSafe codegen

## What changes were proposed in this pull request?
This patch fixes a null handling bug in EqualNullSafe's code generation.

## How was this patch tested?
Updated unit test so they would fail without the fix.

Closes #12628.

Author: Reynold Xin <rxin@databricks.com>
Author: Arash Nabili <arash@levyx.com>

Closes #12799 from rxin/equalnullsafe.

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

@@ -467,7 +467,7 @@ case class EqualNullSafe(left: Expression, right: Expression) extends BinaryComp
     val equalCode = ctx.genEqual(left.dataType, eval1.value, eval2.value)
     ev.copy(code = eval1.code + eval2.code + s"""
         boolean ${ev.value} = (${eval1.isNull} && ${eval2.isNull}) ||
-           (!${eval1.isNull} && $equalCode);""", isNull = "false")
+           (!${eval1.isNull} && !${eval2.isNull} && $equalCode);""", isNull = "false")
   }
 }
 

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/PredicateSuite.scala

@@ -273,7 +273,8 @@ class PredicateSuite extends SparkFunSuite with ExpressionEvalHelper {
   }
 
   test("BinaryComparison: null test") {
-    val normalInt = Literal(1)
+    // Use -1 (default value for codegen) which can trigger some weird bugs, e.g. SPARK-14757
+    val normalInt = Literal(-1)
     val nullInt = Literal.create(null, IntegerType)
 
     def nullTest(op: (Expression, Expression) => Expression): Unit = {