[ONOS-3634] Provide default category name and make url nullable

Change-Id: Id6d0404cc1e46cdd4f54f2ff64ca69313bec9281
diff --git a/core/api/src/main/java/org/onosproject/core/DefaultApplication.java b/core/api/src/main/java/org/onosproject/core/DefaultApplication.java
index e22d14f..5ce5ee7 100644
--- a/core/api/src/main/java/org/onosproject/core/DefaultApplication.java
+++ b/core/api/src/main/java/org/onosproject/core/DefaultApplication.java
@@ -74,7 +74,7 @@
         this.description = checkNotNull(description, "Description cannot be null");
         this.origin = checkNotNull(origin, "Origin cannot be null");
         this.category = checkNotNull(category, "Category cannot be null");
-        this.url = checkNotNull(url, "URL cannot be null");
+        this.url = url;
         this.readme = checkNotNull(readme, "Readme cannot be null");
         this.icon = icon;
         this.role = checkNotNull(role, "Role cannot be null");