Fix NPE when custom JsonAdapter (not nullSafe()) is registered via Supplier

[rob-bygrave]

Fix in DJsonb by adding .nullSafe() for the Supplier<JsonAdapter<T>> case.

Example:

@CustomAdapter(global = false)
public class MoneySerializer2 implements JsonAdapter<BigDecimal> {

  /**  Default constructor -> registered via {@code Supplier<JsonAdapter>} */
  public MoneySerializer2() { }

  @Override
  public BigDecimal fromJson(JsonReader reader) {
    return reader.readDecimal().setScale(2, RoundingMode.DOWN);
  }

  @Override
  public void toJson(JsonWriter writer, BigDecimal value) {
    writer.value(value.setScale(2, RoundingMode.DOWN));
  }
}

Due to using the default constructor, this custom JsonAdapter is registered via Supplier<JsonAdapter<T>> and that wasn't getting the .nullSafe() wrapper.