fdo#72520 : Preserving external data path in chart import.

    - Preserved external data path for chart in newly added
      property for external data.

Change-Id: I3c49093c9e71cd126c097500b3951a345e81039e
Reviewed-on: https://gerrit.libreoffice.org/7214
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>