Recognize the configured "null value" (String) also in reader-infrastructure.

[flappingeagle]

In issue #53 a way to specify a configured "null value" (String) in CsvSchema has been implemented and can be used like:

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

While this new property is already recognized by the Writer-Infrastructure it should also be recognized by the Reader-Infrastructure for deserialization to Java-Null-Values.

[seglo]

I was disappointed to find out the reader doesn't respect this as well. Right now I have no way of determining whether or not a String is a null or empty value.