Move initialization of UnixTimeExprMeta to constructor:

Fixes NVIDIA#5984.

Currently, `UnixTimeExprMeta` postpones part of the initialization of its state
to `tagExprForGpu()`. This leads to situations where an incompletely initialized
`UnixTimeExprMeta` object is exercised at runtime, such as in NVIDIA#5984.

This change moves the initialization to the constructor, leaving `tagExprForGpu()`
to deal only with tagging.

Signed-off-by: MithunR <mythrocks@gmail.com>

sql-plugin/src/main/scala/org/apache/spark/sql/rapids/datetimeExpressions.scala

@@ -374,19 +374,26 @@ abstract class UnixTimeExprMeta[A <: BinaryExpression with TimeZoneAwareExpressi
    rule: DataFromReplacementRule)
   extends BinaryExprMeta[A](expr, conf, parent, rule) {
 
-  var sparkFormat: String = _
-  var strfFormat: String = _
+  val optionalRightStringLit: Option[String] =
+    if (expr.right.dataType == StringType) {
+      extractStringLit(expr.right)
+    } else {
+      None
+    }
+
+  lazy val (sparkFormat, strfFormat): (String, String) =
+    optionalRightStringLit match {
+      case Some(rightLit) =>
+        (rightLit,
+         DateUtils.tagAndGetCudfFormat(this, rightLit,
+                                       expr.left.dataType == DataTypes.StringType))
+      case None => (null, null)
+    }
+
   override def tagExprForGpu(): Unit = {
     // Date and Timestamp work too
-    if (expr.right.dataType == StringType) {
-      extractStringLit(expr.right) match {
-        case Some(rightLit) =>
-          sparkFormat = rightLit
-          strfFormat = DateUtils.tagAndGetCudfFormat(this,
-            sparkFormat, expr.left.dataType == DataTypes.StringType)
-        case None =>
-          willNotWorkOnGpu("format has to be a string literal")
-      }
+    if (expr.right.dataType == StringType && optionalRightStringLit.isEmpty) {
+      willNotWorkOnGpu("format has to be a string literal")
     }
   }
 }