ARROW-241: Add missing implementation for splitAndTransfer in UnionVector

Use simple implementation that actually just copies

Author: StevenMPhillips

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

@@ -264,7 +264,11 @@ public void transfer() {
 
     @Override
     public void splitAndTransfer(int startIndex, int length) {
-
+      to.allocateNew();
+      for (int i = 0; i < length; i++) {
+        to.copyFromSafe(startIndex + i, i, org.apache.arrow.vector.complex.UnionVector.this);
+      }
+      to.getMutator().setValueCount(length);
     }
 
     @Override