Make custom shape export more robust. In case of the test
document, WriteCustomGeometry is called, but this call
does not export anything, and so we get a shape without
any geometry in the DOCX file, which causes problem to MS Word.

Change-Id: Ie7a4e2b8a18bfddaeeb81425ae5f1de04140d43f
Reviewed-on: https://gerrit.libreoffice.org/30241Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>
Tested-by: Tamás Zolnai <tamas.zolnai@collabora.com>