Fixed sparse matrix vector product with matrices that contain empty rows

While a completely empty matrix like the one in the unit test does not appear in practice, there are matrices in practice that contain empty rows, e.g. https://www.cise.ufl.edu/research/sparse/matrices/HB/bcsstm13

Consequently, checking for the padding index is not specific to segments where havePadding returns true, but the paddingIndex must be checked for all formats, including CSR. The paddingIndex is explicitly assigned in multiple cases regardless of the format, e.g. the copySparseToSparseMatrix function drops explicit zeros.