In case a Folder which no longer exists is updated, the server responds
with a GetFolderResponseMessage with ResponseClass="Error". This happens
if the internal cache still contains a folder, but the folder has been
deleted meanwhile (e.g. through the web client).
The message contains an empty Folders element which has to be skipped,
otherwise the outer loop for the GetFolderResponseMessages will try to
traverse the (nonexisting) Folders element on readNextStartElement.
BUG: 387580