Bug in PropertyDeserializer when "enum":null

Hi,
I was parsing a swagger file, and the parse is crashing. By debugging it, I ended up in the class PropertyDeserializer where there is the code

private static List<String> getEnum(JsonNode node, PropertyBuilder.PropertyId type) {
        final List<String> result = new ArrayList<String>();
        JsonNode detailNode = getDetailNode(node, type);
        if (detailNode != null) {
            ArrayNode an = (ArrayNode) detailNode;

this does crash when you have a node with {"enum":null} as the branch is taken (NullNode) in that if statement. A quick fix should be just adding something like "&& detailNode instanceof ArrayNode"

Note: I originally reported this issue at https://github.com/swagger-api/swagger-core/issues/2100#issuecomment-275878181 but was told to open it here

Start in PropertyDeserializer.getEnum, where a swagger node with "enum":null reaches the ArrayNode cast. Reproduce parsing with that input and verify that the parser completes without crashing; add regression coverage if the surrounding project structure provides an appropriate test location.