added UserStore.hasUsers for efficient check whether there are none

Change-Id: Ifacc9c84bda961399b866e860757e8ebe0d78b71

java/com.sap.sse.security.userstore.mongodb/src/com/sap/sse/security/userstore/mongodb/UserStoreImpl.java

@@ -318,6 +318,11 @@ public class UserStoreImpl implements UserStore {
         return new ArrayList<User>(users.values());
     }
 
+    @Override
+    public boolean hasUsers() {
+        return !users.isEmpty();
+    }
+
     @Override
     public User getUserByName(String name) {
         final User result;

java/com.sap.sse.security/src/com/sap/sse/security/UserStore.java

@@ -10,6 +10,8 @@ import com.sap.sse.security.shared.UserManagementException;
 
 public interface UserStore extends Named {
     Iterable<User> getUsers();
+    
+    boolean hasUsers();
 
     /**
      * The user with that {@link User#getName() name} or {@code null} if no such user exists

java/com.sap.sse.security/src/com/sap/sse/security/impl/SecurityServiceImpl.java

@@ -191,7 +191,7 @@ public class SecurityServiceImpl implements ReplicableSecurityService, ClearStat
      * is empty.
      */
     private void initEmptyStore() {
-        if (Util.isEmpty(store.getUsers())) {
+        if (!store.hasUsers()) {
             try {
                 logger.info("No users found, creating default user \"admin\" with password \"admin\"");
                 createSimpleUser("admin", "nobody@sapsailing.com", "admin",