Regression from e8bdd388
(fdo#83751-FILESAVE:Custom Properties dropped while exporting to .pptx,
2014-09-11), there were two problems here:

1) Using the wrong filter when the output is expected to be suitable for
validation.

2) Writing empty custom property values, which is not valid.

Change-Id: Ic18c789c53bd40cc8aa07385cb5fd0d5c7ada6ab