[SPARK-55138][SQL] Fix convertToMapData throwing a NullPointerException

### What changes were proposed in this pull request?
Fix `convertToMapData` throwing a `NullPointerException`. If `NULL` is provided as the value of the options parameter in `from_json`, it will throw a `NullPointerException`, because of `value.toString`.

```
val df = Seq("""{"str": "World"}""").toDS()
val schema = "str STRING"
df.selectExpr(s"from_json(value, '$schema', map('key', 'value', 'mode', NULL))
```

### Why are the changes needed?
Bug fix.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
New unit tests.

### Was this patch authored or co-authored using generative AI tooling?
No.

Closes #53922 from ilicmarkodb/fix_convertToMapData.

Authored-by: ilicmarkodb <marko.ilic@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

Author: ilicmarkodb

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

@@ -66,7 +66,10 @@ object ExprUtils extends EvalHelper with QueryErrorsBase {
         .acceptsType(m.dataType) =>
       val arrayMap = m.eval().asInstanceOf[ArrayBasedMapData]
       ArrayBasedMapData.toScalaMap(arrayMap).map { case (key, value) =>
-        key.toString -> value.toString
+        if (key == null) {
+          throw QueryExecutionErrors.nullAsMapKeyNotAllowedError()
+        }
+        key.toString -> (if (value == null) "null" else value.toString)
       }
     case m: CreateMap =>
       throw QueryCompilationErrors.keyValueInMapNotStringError(m)

sql/core/src/test/scala/org/apache/spark/sql/JsonFunctionsSuite.scala

@@ -365,6 +365,32 @@ class JsonFunctionsSuite extends QueryTest with SharedSparkSession {
       Row(Row(1, "haa")) :: Nil)
   }
 
+  test("from_json with NULL in options map values") {
+    val df = Seq("""{"str": "World"}""").toDS()
+    val schema = "str STRING"
+
+    checkAnswer(
+      df.selectExpr(
+        s"from_json(value, '$schema', map('key', 'value', 'mode', NULL))"
+      ),
+      Row(Row("World")) :: Nil
+    )
+  }
+
+  test("from_json with NULL in options map key") {
+    val df = Seq("""{"str": "World"}""").toDS()
+    val schema = "str STRING"
+
+    checkError(
+      exception = intercept[SparkRuntimeException] {
+        df.selectExpr(
+          s"from_json(value, '$schema', map('mode', 'PERMISSIVE', NULL, 'value'))"
+        ).show()
+      },
+      condition = "NULL_MAP_KEY"
+    )
+  }
+
   test("to_json - struct") {
     val df = Seq(Tuple1(Tuple1(1))).toDF("a")