[SPARK-33756][SQL] Make BytesToBytesMap's MapIterator idempotent

### What changes were proposed in this pull request?
Make MapIterator of BytesToBytesMap `hasNext` method idempotent

### Why are the changes needed?
The `hasNext` maybe called multiple times, if not guarded, second call of hasNext method after reaching the end of iterator will throw NoSuchElement exception.

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

### How was this patch tested?
Update a unit test to cover this case.

Closes #30728 from advancedxy/SPARK-33756.

Authored-by: Xianjin YE <advancedxy@gmail.com>
Signed-off-by: Sean Owen <srowen@gmail.com>
(cherry picked from commit 1339168)
Signed-off-by: Sean Owen <srowen@gmail.com>

core/src/main/java/org/apache/spark/unsafe/map/BytesToBytesMap.java

@@ -408,10 +408,12 @@ public void remove() {
     }
 
     private void handleFailedDelete() {
-      // remove the spill file from disk
-      File file = spillWriters.removeFirst().getFile();
-      if (file != null && file.exists() && !file.delete()) {
-        logger.error("Was unable to delete spill file {}", file.getAbsolutePath());
+      if (spillWriters.size() > 0) {
+        // remove the spill file from disk
+        File file = spillWriters.removeFirst().getFile();
+        if (file != null && file.exists() && !file.delete()) {
+          logger.error("Was unable to delete spill file {}", file.getAbsolutePath());
+        }
       }
     }
   }

core/src/test/java/org/apache/spark/unsafe/map/AbstractBytesToBytesMapSuite.java

@@ -560,6 +560,8 @@ public void spillInIterator() throws IOException {
         iter2.next();
       }
       assertFalse(iter2.hasNext());
+      // calls hasNext twice deliberately, make sure it's idempotent
+      assertFalse(iter2.hasNext());
     } finally {
       map.free();
       for (File spillFile : spillFilesCreated) {