Per [FasterXML#106] added allowNullFields to provide the option of pr…

…ocessing null elements of lists as empty fields or the nullCharacter during serialization instead of the default behavior which skips null elements.

src/test/java/com/fasterxml/jackson/dataformat/csv/ser/NullWritingTest.java

@@ -93,6 +93,14 @@ public void testCustomNullValueInObjectListIssue106() throws Exception
 
         String result = mapper.writer(schema).writeValueAsString(list);
         assertEquals("d0,n/a,d2\n", result);
+
+        list = Arrays.asList(null, "d1", "d2");
+        result = mapper.writer(schema).writeValueAsString(list);
+        assertEquals("n/a,d1,d2\n", result);
+
+        list = Arrays.asList("d0", "d1", null);
+        result = mapper.writer(schema).writeValueAsString(list);
+        assertEquals("d0,d1,n/a\n", result);
     }
 
     public void testDefaultNullValueInObjectListIssue106() throws Exception
@@ -104,8 +112,15 @@ public void testDefaultNullValueInObjectListIssue106() throws Exception
             .build();
 
         List list = Arrays.asList("d0", null, "d2");
-
         String result = mapper.writer(schema).writeValueAsString(list);
         assertEquals("d0,,d2\n", result);
+
+        list = Arrays.asList(null, "d1", "d2");
+        result = mapper.writer(schema).writeValueAsString(list);
+        assertEquals(",d1,d2\n", result);
+
+        list = Arrays.asList("d0", "d1", null);
+        result = mapper.writer(schema).writeValueAsString(list);
+        assertEquals("d0,d1,\n", result);
     }
 }