Avoid NPE in AdditionalPropertiesValidator (#424)

additionalPropertiesSchema can be null by some code paths, but preloadJsonSchema assumes it's always non-null.

Author: flozano

src/main/java/com/networknt/schema/AdditionalPropertiesValidator.java

@@ -108,6 +108,8 @@ public Set<ValidationMessage> validate(JsonNode node, JsonNode rootNode, String
 
     @Override
     public void preloadJsonSchema() {
-        additionalPropertiesSchema.initializeValidators();
+        if(additionalPropertiesSchema!=null) {
+            additionalPropertiesSchema.initializeValidators();
+        }
     }
 }