Added a null check to prevent a NullPointerException to be thrown in

getUserByEmail if there is no user for a given email address

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

@@ -320,7 +320,7 @@ public class UserStoreImpl implements UserStore {
             result = null;
         } else {
             Set<User> set = usersByEmail.get(email);
-            if (set.isEmpty()) {
+            if (set == null || set.isEmpty()) {
                 result = null;
             } else {
                 result = set.iterator().next();