Minor changes to UserDatabase. Added changeable interface just to make the testing work

Author: Madhavan7

src/main/java/Changeable.java

@@ -0,0 +1,3 @@
+public interface Changeable {
+    void changeFeature(String feature, String newFeature);
+}

src/main/java/UserDatabase.java

@@ -15,6 +15,7 @@ public boolean UserExists(String username, String email) {
                 try{
                     user = (User)in.readObject();
                 }catch(NullPointerException e){
+                    user = null;
                     break;
                 }
             }while(!user.getEmail().equals(email) && !user.getUsername().equals(username));