Prevent errors when FileSystemBackend::getFile() returns NULL

This is a followup of #3082271: TypeError: Argument 1 passed to ::prepareItem() must be an instance of stdClass, bool given. In that ticket a bug was fixed where FileSystemBackend::getFile() could return FALSE when a file could not be serialized, where the function should only return a cache object or NULL.

I have now checked all calls to the ::getFile() methods and have identified two methods that do not handle the case where the return value is NULL. We should fix these so that they do not throw fatal errors in case the file being read is deleted or contains something else than a serialized cache object.