virtual_media: Fix for bmcweb crash

bmcweb crashes when user not providing userName or password while
posting ISO form Redfish.

This fix provides to avoid bmcweb crash when user try to post ISO images
from Redfish without providing username or password.

Tested:
Redfish validator passed with this change.
Verified by Posting ISO from Redfish.

Signed-off-by: Arun Kapale <arunx.kapale@intel.com>
Signed-off-by: Jayaprakash Mutyala <mutyalax.jayaprakash@intel.com>
Change-Id: Ib9c7ed668f13943bdacdd9afda0e65d6bcc438d0