Missing `null` writes when writing `Object[]` values

[cowtowncoder]

(moved from https://github.com/FasterXML/jackson-dataformat-csv/issues/116 by @georgewfraser)

When writing arrays to CSV, if there is a null, columns get written out of order. Reproduced in 2.7.1 with this test: https://gist.github.com/georgewfraser/164647fbaca61aadf9ee :

    @Test
    public void objectArray() throws IOException {
        CsvSchema schema = CsvSchema.builder()
                                    .addColumn("a", CsvSchema.ColumnType.NUMBER)
                                    .addColumn("b", CsvSchema.ColumnType.NUMBER)
                                    .setUseHeader(true)
                                    .build();
        ObjectWriter writer = Config.CSV.writer(schema);
        StringWriter out = new StringWriter();
        SequenceWriter sequence = writer.writeValues(out);

        sequence.write(new Object[]{ 1, 2 });
        sequence.write(new Object[]{ null, 2 });
        sequence.write(new Object[]{ null, null });
        sequence.write(new Object[]{ 1, null });

        assertEquals("\"a\",\"b\"\n" +
                     "1,2\n" +
                     ",2\n" +
                     ",\n" +
                     "1,\n", out.toString());
    }

[cowtowncoder]

More likely related to #106: looks like nulls for "array in array" are suppressed. This could be somewhat problematic even for other entries since "null value" would not get written.

[daliclass]

@cowtowncoder Hey, Is anyone working on this I would be happy to pick it up

Added a gist for the current behaviour in 2.9.0 and what I think the desired behaviour should be

https://gist.github.com/daliclass/19507eb79939e6ad716b30cd241c612b

[ganchurin]

@cowtowncoder Is there any workaround for the issue?

[cowtowncoder]

@ganchurin not aware of work-arounds unfortunately. If anyone has time to look into this, I would be happy to help validate fix, integrate, but right now I do not have time in short term. I will add this as something to (re-)evaluate tho as it is an important issue.

[gimiz]

Workaround: Before writing change null value to an empty string. In not-quoted CSV empty string looks the same as null (separator only). If you need to use qouteChar, set CsvGenerator.Feature.ALWAYS_QUOTE_STRINGS to false to use quotes only when necessary. Empty string will stay not-quoted. If you need all values quoted, use another library.