checking for null additional names.

fixed #12.

Author: andrejserafim

core/src/main/java/org/everit/json/schema/ObjectSchema.java

@@ -255,10 +255,15 @@ private void testAdditionalProperties(final JSONObject subject) {
   }
 
   private Stream<String> getAdditionalProperties(final JSONObject subject) {
-    return Arrays
-        .stream(JSONObject.getNames(subject))
-        .filter(key -> !propertySchemas.containsKey(key))
-        .filter(key -> !matchesAnyPattern(key));
+    String[] names = JSONObject.getNames(subject);
+    if(names == null) {
+      return Stream.empty();
+    } else {
+      return Arrays
+              .stream(names)
+              .filter(key -> !propertySchemas.containsKey(key))
+              .filter(key -> !matchesAnyPattern(key));
+    }
   }
 
   private void testProperties(final JSONObject subject) {

tests/src/test/java/org/everit/json/schema/EmptyObjectTest.java

@@ -0,0 +1,24 @@
+package org.everit.json.schema;
+
+import org.everit.json.schema.loader.SchemaLoader;
+import org.json.JSONObject;
+import org.json.JSONTokener;
+import org.junit.Test;
+
+public class EmptyObjectTest {
+    @Test
+    public void validateEmptyObject() {
+
+        JSONObject jsonSchema = new JSONObject(new JSONTokener(
+                MetaSchemaTest.class
+                        .getResourceAsStream("/org/everit/json/schema/json-schema-draft-04.json")));
+
+        JSONObject jsonSubject = new JSONObject("{\n" +
+                "  \"type\": \"object\",\n" +
+                "  \"properties\": {}\n" +
+                "}");
+
+        Schema schema = SchemaLoader.load(jsonSchema);
+        schema.validate(jsonSubject);
+    }
+}