fix: provide fallback reasons for Generate and ArrayFilter operators

When CometExplodeExec.convert was called with an empty childOp (child
operator not native), it propagated tags from output attributes which
never have explain info, resulting in "Generate [COMET: ]" with no
reason. Split the check to provide an explicit message for the
non-native child case. Also add a missing reason to
CometArrayFilter.getSupportLevel for unsupported filter functions.

Author: andygrove

spark/src/main/scala/org/apache/comet/serde/arrays.scala

@@ -622,7 +622,8 @@ object CometArrayFilter extends CometExpressionSerde[ArrayFilter] {
   override def getSupportLevel(expr: ArrayFilter): SupportLevel = {
     expr.function.children.headOption match {
       case Some(_: IsNotNull) => Compatible()
-      case _ => Unsupported()
+      case _ =>
+        Unsupported(Some("Only array_compact (ArrayFilter with IsNotNull) is supported"))
     }
   }
 

spark/src/main/scala/org/apache/spark/sql/comet/operators.scala

@@ -1217,7 +1217,12 @@ object CometExplodeExec extends CometOperatorSerde[GenerateExec] {
       exprToProto(attr, op.child.output)
     }
 
-    if (projectExprs.exists(_.isEmpty) || childOp.isEmpty) {
+    if (childOp.isEmpty) {
+      withInfo(op, s"${op.nodeName} is not native because the child operator is not native")
+      return None
+    }
+
+    if (projectExprs.exists(_.isEmpty)) {
       withInfo(op, op.output: _*)
       return None
     }