To indicate why the result isn't available ("External content
disabled") and in future maybe signal to the formula cell that it
could keep the hybrid string result, see source code comment.

Change-Id: Ic5d336b8489e8776f7b640b7e46815e71d0a82a4
Reviewed-on: https://gerrit.libreoffice.org/57738Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Jenkins