[SPARK-16582][SQL] Explicitly define isNull = false for non-nullable expressions

## What changes were proposed in this pull request?

This patch is just a slightly safer way to fix the issue we encountered in https://github.com/apache/spark/pull/14168 should this pattern re-occur at other places in the code.

## How was this patch tested?

Existing tests. Also, I manually tested that it fixes the problem in SPARK-16514 without having the proposed change in https://github.com/apache/spark/pull/14168

Author: Sameer Agarwal <sameerag@cs.berkeley.edu>

Closes #14227 from sameeragarwal/codegen.

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

@@ -377,6 +377,7 @@ abstract class UnaryExpression extends Expression {
       """)
     } else {
       ev.copy(code = s"""
+        boolean ${ev.isNull} = false;
         ${childGen.code}
         ${ctx.javaType(dataType)} ${ev.value} = ${ctx.defaultValue(dataType)};
         $resultCode""", isNull = "false")
@@ -475,6 +476,7 @@ abstract class BinaryExpression extends Expression {
       """)
     } else {
       ev.copy(code = s"""
+        boolean ${ev.isNull} = false;
         ${leftGen.code}
         ${rightGen.code}
         ${ctx.javaType(dataType)} ${ev.value} = ${ctx.defaultValue(dataType)};
@@ -617,6 +619,7 @@ abstract class TernaryExpression extends Expression {
         $nullSafeEval""")
     } else {
       ev.copy(code = s"""
+        boolean ${ev.isNull} = false;
         ${leftGen.code}
         ${midGen.code}
         ${rightGen.code}