Add a way to specify "null value" (String) for `CsvGenerator` to use when writing `null`s

[cowtowncoder]

Looks like many CSV processors allow use of specific placeholder to use instead of Java null, instead of empty String. We should allow that as well; and with 2.5, may be able to configure it nicely on per-call basis via ObjectWriter as well.

[cowtowncoder]

Implemented; used like:

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

[flappingeagle]

nice, we will use this to serialize null-values as "(null)" like the DBVisualizer-App does it ;)

It would be nice if this configuration also works for deserialization. For example, the following call would not recognize this configuration and will return the String "(null) " in the resulting map while parsing a csv-file:

        Map<String, String> line = csvReader.nextValue();

but its also no problem for us to post-process this ourselves...

[cowtowncoder]

Good idea; I agree in that this would make sense. Could you file a separate issue so this is easier to track?