This reverts commit 2757ee9fe610e253e4ccc37423fa420004d0f388. Besides causing a performance regression, I now notice that there is code in SW that relies on iterating over two different SfxItemSet's in parallel, and assumes that missing items are returned as nullptr, which is not the case for my std::map based change. Change-Id: I2b1110350fe4c4b74e5508558e9661ef1e1a103e