added unittest with null permitted value to ConstSchemaTest

Author: erosb

core/src/test/java/org/everit/json/schema/ConstSchemaTest.java

@@ -64,6 +64,13 @@ public void toStringTest() {
         assertEquals("{\"const\":true}", actual);
     }
 
+    @Test
+    public void toStringWithNull() {
+        ConstSchema subject = ConstSchema.builder().permittedValue(null).build();
+        String actual = subject.toString();
+        assertEquals("{\"const\":null}", actual);
+    }
+
     @Test
     public void toStringWithObject() {
         ConstSchema subject = ConstSchema.builder().permittedValue(new JSONObject("{\"a\":2}")).build();