Null fields are always ignored when serializing list of objects

[morbrian]

When serializing a list of objects, any null values are skipped over as if they do not exist instead of writing an empty character or using a custom value in place of null.

Example 1 (Expected Default Behavior)

List list = Arrays.asList("d0", null, "d2");

Expected:

    d0,,d2

Actual:

    d0,d2

Example 2 (Expected Custom Behavior)

List list = Arrays.asList("d0", null, "d2");

Assuming a schema like: 

CsvSchema.builder().setNullValue("n/a").build();

Expected:

    d0,n/a,d2

Actual:

    d0,d2

Tested Versions

Versions: 2.6.3 - 2.7.0-rc2

Unit Tests

To demonstrate the issue, the test methods below may be added to NullWritingTest.java

public void testCustomNullValueInObjectList() throws Exception
{
    ObjectMapper mapper = mapperForCsv();
    CsvSchema schema = CsvSchema.builder()
        .setNullValue("n/a")
        .build();

    List list = Arrays.asList("d0", null, "d2");

    String result = mapper.writer(schema).writeValueAsString(list);
    // MUST use doubling for quotes!
    assertEquals("d0,n/a,d2\n", result);
}

public void testDefaultNullValueInObjectList() throws Exception
{
    ObjectMapper mapper = mapperForCsv();
    CsvSchema schema = CsvSchema.builder().build();

    List list = Arrays.asList("d0", null, "d2");

    String result = mapper.writer(schema).writeValueAsString(list);
    // MUST use doubling for quotes!
    assertEquals("d0,,d2\n", result);
}

Other Background

This issue may be related to an overlooked area of CsvGenerator.java when fixing #83 or #69

In that class, there are some related notes in method public void writeNull() throws IOException, the second condition in the block below is matched, but the write activity is commented out:

if (_arraySeparator >= 0) {
    _addToArray(_schema.getNullValueOrEmpty());
} else if (!_writeContext.inObject()) { // as per [#69]
    // note: 'root' not enough, for case of wrap-as array, or serialize List

    // or, to write 'empty Object' (for common case), would
    // write single null, then finish row, like so:
    /*
    _writer.writeNull(_columnIndex());
    finishRow();
   */
} else {
    _writer.writeNull(_columnIndex()); 
}

[cowtowncoder]

@morbrian Quick question: can you reproduce this with 2.7.0-rc2?

[morbrian]

@cowtowncoder Yes, I tried 2.6.3, 2.6.4, 2.7.0-rc2, as well as the current master.

As I play with the code I am learning the issue may already be known about, but perhaps not easy to fix without impacting other desired behavior.

Uncommenting the code block described in the issue report fixes the problem for my current use cases, but doing so breaks other related Null tests: testNullToStream() and testNullToString()

The testNullToStream() test has comments saying nulls are ignored for now.