A regression of 1166966e.

We need to consider 2 use-cases:

1. Protecting an existing document with a password (by overwriting
   the original file). In this case we don't want to generate a
   thumbnail from the now encrypted file, but we do want to erase
   the stored thumbnail and show a generic icon instead.

2. Closing a modified document without saving the changes. Here we
   don't want to generate a thumbnail either, because it may
   contain the unsaved changes, but either we don't want to replace
   the stored thumbnail, because most likely it correctly represents
   the last saved state of the document.

Change-Id: Ia7b1f3dbc9fbbc2ef1d87442c1dee25306f65826
Reviewed-on: https://gerrit.libreoffice.org/16659Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Maxim Monastirsky <momonasmon@gmail.com>