[SPARK-49480][CORE] Fix NullPointerException from `SparkThrowableHelp…

…er.isInternalError`

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

Handle null input for `SparkThrowableHelper.isInternalError` method.

### Why are the changes needed?

The `SparkThrowableHelper.isInternalError` method doesn't handle null input, and it could lead to NullPointerException. It happens when a `SparkException` without `errorClass` is invoked `isInternalError`.

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

No

### How was this patch tested?

Add 2 assertions to current test cases to cover this issue.

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

No

Closes apache#47946 from jshmchenxi/SPARK-49480/null-pointer-is-internal-error.

Authored-by: Xi Chen <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

common/utils/src/main/scala/org/apache/spark/SparkThrowableHelper.scala

@@ -74,7 +74,7 @@ private[spark] object SparkThrowableHelper {
   }
 
   def isInternalError(errorClass: String): Boolean = {
-    errorClass.startsWith("INTERNAL_ERROR")
+    errorClass != null && errorClass.startsWith("INTERNAL_ERROR")
   }
 
   def getMessage(e: SparkThrowable with Throwable, format: ErrorMessageFormat.Value): String = {

core/src/test/scala/org/apache/spark/SparkThrowableSuite.scala

@@ -259,6 +259,7 @@ class SparkThrowableSuite extends SparkFunSuite {
     } catch {
       case e: SparkThrowable =>
         assert(e.getErrorClass == null)
+        assert(!e.isInternalError)
         assert(e.getSqlState == null)
       case _: Throwable =>
         // Should not end up here
@@ -275,6 +276,7 @@ class SparkThrowableSuite extends SparkFunSuite {
     } catch {
       case e: SparkThrowable =>
         assert(e.getErrorClass == "CANNOT_PARSE_DECIMAL")
+        assert(!e.isInternalError)
         assert(e.getSqlState == "22018")
       case _: Throwable =>
         // Should not end up here