Add null-checking to addRequired

Author: xhh

modules/swagger-models/src/main/java/io/swagger/models/ModelImpl.java

@@ -140,7 +140,10 @@ public void setFormat(String format) {
     }
 
     public void addRequired(String name) {
-        this.required.add(name);
+        if (required == null) {
+            required = new ArrayList<String>();
+        }
+        required.add(name);
         Property p = properties.get(name);
         if (p != null) {
             p.setRequired(true);