oox: fix a few bogus SAL_WARN

The idea is that when an element is unhandled, it's either unhandled
explicitly by an empty "case..break" block (so grep can quickly find it)
or we raise a warning. Add a few placeholders for cases that we won't
probably handle better in the near future to silence warnings.

Change-Id: I41dec7802c4671550130393b4f98a8f0d8dfc059