[SPARK-25036][SQL] Should compare ExprValue.isNull with LiteralTrue/LiteralFalse

## What changes were proposed in this pull request?

This PR fixes a comparison of `ExprValue.isNull` with `String`. `ExprValue.isNull` should be compared with `LiteralTrue` or `LiteralFalse`.

This causes the following compilation error using scala-2.12 with sbt. In addition, this code may also generate incorrect code in Spark 2.3.

```
/home/ishizaki/Spark/PR/scala212/spark/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/stringExpressions.scala:94: org.apache.spark.sql.catalyst.expressions.codegen.ExprValue and String are unrelated: they will most likely always compare unequal
[error] [warn]         if (eval.isNull != "true") {
[error] [warn]
[error] [warn] /home/ishizaki/Spark/PR/scala212/spark/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/stringExpressions.scala:126: org.apache.spark.sql.catalyst.expressions.codegen.ExprValue and String are unrelated: they will most likely never compare equal
[error] [warn]              if (eval.isNull == "true") {
[error] [warn]
[error] [warn] /home/ishizaki/Spark/PR/scala212/spark/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/stringExpressions.scala:133: org.apache.spark.sql.catalyst.expressions.codegen.ExprValue and String are unrelated: they will most likely never compare equal
[error] [warn]             if (eval.isNull == "true") {
[error] [warn]
[error] [warn] /home/ishizaki/Spark/PR/scala212/spark/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/GenerateUnsafeProjection.scala:90: org.apache.spark.sql.catalyst.expressions.codegen.ExprValue and String are unrelated: they will most likely never compare equal
[error] [warn]       if (inputs.map(_.isNull).forall(_ == "false")) {
[error] [warn]
```

## How was this patch tested?

Existing UTs

Author: Kazuaki Ishizaki <[email protected]>

Closes apache#22012 from kiszk/SPARK-25036a.

Author: kiszk

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/GenerateUnsafeProjection.scala

@@ -87,7 +87,7 @@ object GenerateUnsafeProjection extends CodeGenerator[Seq[Expression], UnsafePro
       // For top level row writer, it always writes to the beginning of the global buffer holder,
       // which means its fixed-size region always in the same position, so we don't need to call
       // `reset` to set up its fixed-size region every time.
-      if (inputs.map(_.isNull).forall(_ == "false")) {
+      if (inputs.map(_.isNull).forall(_ == FalseLiteral)) {
         // If all fields are not nullable, which means the null bits never changes, then we don't
         // need to clear it out every time.
         ""

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

@@ -91,7 +91,7 @@ case class ConcatWs(children: Seq[Expression])
       val args = ctx.freshName("args")
 
       val inputs = strings.zipWithIndex.map { case (eval, index) =>
-        if (eval.isNull != "true") {
+        if (eval.isNull != TrueLiteral) {
           s"""
              ${eval.code}
              if (!${eval.isNull}) {
@@ -123,14 +123,14 @@ case class ConcatWs(children: Seq[Expression])
         child.dataType match {
           case StringType =>
             ("", // we count all the StringType arguments num at once below.
-             if (eval.isNull == "true") {
+             if (eval.isNull == TrueLiteral) {
                ""
              } else {
                s"$array[$idxVararg ++] = ${eval.isNull} ? (UTF8String) null : ${eval.value};"
              })
           case _: ArrayType =>
             val size = ctx.freshName("n")
-            if (eval.isNull == "true") {
+            if (eval.isNull == TrueLiteral) {
               ("", "")
             } else {
               (s"""