fix issue swagger-api#1143: add another null check.

The problem was trying to iterate over a null list.

Author: ePaul

modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/JavaClientCodegen.java

@@ -331,6 +331,8 @@ public Map<String, Object> postProcessModels(Map<String, Object> objs) {
                     continue;
                 List<String> values = (List<String>) allowableValues.get("values");
                 // put "enumVars" map into `allowableValues", including `name` and `value`
+                if (values == null)
+                    continue;
                 List<Map<String, String>> enumVars = new ArrayList<Map<String, String>>();
                 for (String value : values) {
                     Map<String, String> enumVar = new HashMap<String, String>();