fix(auth): null-check username when getting current user (#1490)

Author: raphkim

aws-auth-cognito/src/main/java/com/amplifyframework/auth/cognito/AWSCognitoAuthPlugin.java

@@ -1053,7 +1053,7 @@ public void onError(Exception error) {
 
     @Override
     public AuthUser getCurrentUser() {
-        if (userId != null) {
+        if (userId != null && awsMobileClient.getUsername() != null) {
             return new AuthUser(userId, awsMobileClient.getUsername());
         } else {
             return null;