Work around new calls to GetFilter()->GetMimeType() on null filter
Work around new calls to GetFilter()->GetMimeType() on null filter

...introduced with 75bc673b60d3eb40713868ce3d1b4f62dd6e1208 "Travelled MIME type
through UCB for CMIS UCP to use it when creating documents" and causing crashes
in sc_unoapi test.  Many SfxMedium instances appear to have a null filter, so
this workaround at least fixes the crashes.  No idea what a good fix should look
like, so will ask Cedric to review his commit.

Change-Id: I27180265b6c79c7ed5f459b03c9e188ff6a0f56d

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core