fdo#77140 embeddedobj: avoid unneeded conversion in ConvertBufferToFormat()

The primary reason is not performance: on export currently we loose some
EMF+ properties. While it would be good to fix all those problems one by
one, this is a small fix to avoid all kind of roundtrip problems in this
situation.

Change-Id: If5e1c1eabd8290f36b538e374c9707ae17097786
(cherry picked from commit 2a7fdf2a7bb7345c49c988dfd1fcdb41cebb8a1e)
Reviewed-on: https://gerrit.libreoffice.org/8903
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>