[SPARK-48150][SQL] try_parse_json output should be declared as nullable

### What changes were proposed in this pull request?

The `try_parse_json` expression added in #46141 declares improper output nullability: the `try_` version's output must be marked as nullable. This PR corrects the nullability and adds a test.

### Why are the changes needed?

Incorrectly declaring an expression's output as non-nullable when it is actually nullable may lead to crashes.

### Does this PR introduce _any_ user-facing change?

Yes, it affects output nullability and thus may affect query result schemas.

### How was this patch tested?

New unit test cases.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #46409 from JoshRosen/fix-try-parse-json-nullability.

Authored-by: Josh Rosen <joshrosen@databricks.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

connector/connect/common/src/test/resources/query-tests/explain-results/function_try_parse_json.explain

@@ -1,2 +1,2 @@
-Project [staticinvoke(class org.apache.spark.sql.catalyst.expressions.variant.VariantExpressionEvalUtils$, VariantType, parseJson, g#0, false, StringType, BooleanType, true, false, true) AS try_parse_json(g)#0]
+Project [staticinvoke(class org.apache.spark.sql.catalyst.expressions.variant.VariantExpressionEvalUtils$, VariantType, parseJson, g#0, false, StringType, BooleanType, true, true, true) AS try_parse_json(g)#0]
 +- LocalRelation <empty>, [id#0L, a#0, b#0, d#0, e#0, f#0, g#0]

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/variant/variantExpressions.scala

@@ -59,7 +59,7 @@ case class ParseJson(child: Expression, failOnError: Boolean = true)
     "parseJson",
     Seq(child, Literal(failOnError, BooleanType)),
     inputTypes :+ BooleanType,
-    returnNullable = false)
+    returnNullable = !failOnError)
 
   override def inputTypes: Seq[AbstractDataType] = StringType :: Nil
 

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/variant/VariantExpressionSuite.scala

@@ -810,6 +810,15 @@ class VariantExpressionSuite extends SparkFunSuite with ExpressionEvalHelper {
       "Hello")
   }
 
+  test("SPARK-48150: ParseJson expression nullability") {
+    assert(!ParseJson(Literal("["), failOnError = true).replacement.nullable)
+    assert(ParseJson(Literal("["), failOnError = false).replacement.nullable)
+    checkEvaluation(
+      ParseJson(Literal("["), failOnError = false).replacement,
+      null
+    )
+  }
+
   test("cast to variant") {
     def check[T : TypeTag](input: T, expectedJson: String): Unit = {
       val cast = Cast(Literal.create(input), VariantType, evalMode = EvalMode.ANSI)