GH-399: Check for null writers in DenseUnionWriter#setPosition (#938)

## GH-399 Fix setPosition fails with NullPointerException

Fixed a `NullPointerException` in `DenseUnionWriter#setPosition`.

The issue was that `setPosition` tried to update all writers in its
internal array, even if they hadn't been initialized yet. I added a null
check so it only updates writers that actually exist.

Also added a regression test (`TestDenseUnionWriterNPE`) to verify the
fix and updated `.gitignore`.

Closes #399.

Author: Kaustav-Sarkar

.gitignore

@@ -7,10 +7,12 @@
 .buildpath
 .checkstyle
 .classpath
+.cursor/
 .factorypath
 .idea/
 .project
 .settings/
+.vscode/
 /*-build/
 /.mvn/.develocity/
 /apache-arrow-java-*

vector/src/main/codegen/templates/DenseUnionWriter.java

@@ -55,7 +55,9 @@ public DenseUnionWriter(DenseUnionVector vector, NullableStructWriterFactory nul
   public void setPosition(int index) {
     super.setPosition(index);
     for (BaseWriter writer : writers) {
-      writer.setPosition(index);
+      if (writer != null) {
+        writer.setPosition(index);
+      }
     }
   }
 

vector/src/test/java/org/apache/arrow/vector/complex/writer/TestComplexWriter.java

@@ -2530,4 +2530,47 @@ public void extensionWriterReader() throws Exception {
       }
     }
   }
+
+  @Test
+  void testListOfDenseUnionWriterNPE() {
+    // Regression test for https://github.com/apache/arrow-java/issues/399
+    try (ListVector listVector = ListVector.empty("list", allocator)) {
+      listVector.addOrGetVector(FieldType.nullable(MinorType.DENSEUNION.getType()));
+      UnionListWriter listWriter = listVector.getWriter();
+
+      listWriter.startList();
+      listWriter.endList();
+    }
+  }
+
+  @Test
+  void testListOfDenseUnionWriterWithData() {
+    try (ListVector listVector = ListVector.empty("list", allocator)) {
+      listVector.addOrGetVector(FieldType.nullable(MinorType.DENSEUNION.getType()));
+
+      UnionListWriter listWriter = listVector.getWriter();
+      listWriter.startList();
+      listWriter.writeInt(100);
+      listWriter.writeBigInt(200L);
+      listWriter.endList();
+
+      listWriter.startList();
+      listWriter.writeFloat4(3.14f);
+      listWriter.endList();
+
+      listVector.setValueCount(2);
+
+      assertEquals(2, listVector.getValueCount());
+
+      List<?> value0 = (List<?>) listVector.getObject(0);
+      List<?> value1 = (List<?>) listVector.getObject(1);
+
+      assertEquals(2, value0.size());
+      assertEquals(100, value0.get(0));
+      assertEquals(200L, value0.get(1));
+
+      assertEquals(1, value1.size());
+      assertEquals(3.14f, value1.get(0));
+    }
+  }
 }