Additional properties serializer issues with the framework

[akomisarek]

@MartinYSpasov / @allanmckenzie

When there are no additional properties in the request, the additionalProperties map in POJO is null. This leads to NPE in AdditionalPropertiesSerializer.

Code which can be used to reproduce the issue is:

    public static void main(String[] args) throws JsonProcessingException {
        final ObjectMapper objectMapper = new ObjectMapperProducer().objectMapper();

        final ClassWithAdditionalProperties classWithAdditionalProperties = new ClassWithAdditionalProperties(3, null);
        final String serializedClassWithAdditionalProperties = objectMapper.writeValueAsString(classWithAdditionalProperties);
        System.out.println(serializedClassWithAdditionalProperties);
    }

    public static class ClassWithAdditionalProperties {
        private final int value;
        private final Map<String, Object> additionalProperties;

        public ClassWithAdditionalProperties(final int value, final Map<String, Object> additionalProperties) {
            this.value = value;
            this.additionalProperties = additionalProperties;
        }

        public int getValue() {
            return value;
        }

        public Map<String, Object> getAdditionalProperties() {
            return additionalProperties;
        }
    }

Not sure if it is better to defend against NPE (probably would work, why not?) or change the framework in another place to set the empty map...