fdo#52076 remove Library dir when no library

when saving to same storage (in place)
we already did that when empty "Standard" was the only library,
but not when there was no library...
Cannot get through that state in through UI, but possible through scripting.

Change-Id: I0f1129e034ac31b3eaf6bd388a03aee5aae5b87e
Reviewed-on: https://gerrit.libreoffice.org/11276
Reviewed-by: Muthu Subramanian K <muthusuba@gmail.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>