Do not allow skipNull() on nullable producer

Author: martin-traverse

adapter/avro/src/main/java/org/apache/arrow/adapter/avro/producers/AvroNullableProducer.java

@@ -48,8 +48,9 @@ public void produce(Encoder encoder) throws IOException {
 
   @Override
   public void skipNull() {
-    delegate.skipNull();
-    super.skipNull();
+    // Should never be called on nullable producer
+    // Calling produce() will skipNull() on the delegate
+    throw new UnsupportedOperationException();
   }
 
   @Override