Fix bmcweb crash problem when no-auth

After insecure-disable-auth=enabled. it is not needed to do login and
establish session before request.
GET /redfish/v1/AccountService/Accounts won't get any status code and
cause the bmcweb service crashed.
Set the default string value for `thisUser`.

Assign the user only if the session is not nullptr.

Test:
Use no-auth:

curl -k -X GET https://<bmcip>/redfish/v1/AccountService/Accounts
{
  "@odata.id": "/redfish/v1/AccountService/Accounts",
  "@odata.type": "#ManagerAccountCollection.ManagerAccountCollection",
  "Description": "BMC User Accounts",
  "Members": [],
  "Members@odata.count": 0,
  "Name": "Accounts Collection"
}

Use basic-auth:

curl -k -u root:0penBmc -X GET
https://<bmcip>/redfish/v1/AccountService/Accounts
{
  "@odata.id": "/redfish/v1/AccountService/Accounts",
  "@odata.type": "#ManagerAccountCollection.ManagerAccountCollection",
  "Description": "BMC User Accounts",
  "Members": [
    {
      "@odata.id": "/redfish/v1/AccountService/Accounts/root"
    }
  ],
  "Members@odata.count": 1,
  "Name": "Accounts Collection"
}

Signed-off-by: JunLin Chen <Jun-Lin.Chen@quantatw.com>
Change-Id: Ifa9844c4dbc2f172338b24fba7a09ae013b6d473
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/redfish-core/lib/account_service.hpp b/redfish-core/lib/account_service.hpp
index 8890ae1..d59c8c2 100644
--- a/redfish-core/lib/account_service.hpp
+++ b/redfish-core/lib/account_service.hpp
@@ -1501,8 +1501,11 @@
                 Privileges effectiveUserPrivileges =
                     redfish::getUserPrivileges(req.userRole);
 
-                std::string thisUser = req.session->username;
-
+                std::string thisUser;
+                if (req.session)
+                {
+                    thisUser = req.session->username;
+                }
                 crow::connections::systemBus->async_method_call(
                     [asyncResp, thisUser, effectiveUserPrivileges](
                         const boost::system::error_code ec,