Fix dot*w to return 0 for empty vectors (BLAS interface)
Fix dot*w to return 0 for empty vectors (BLAS interface)

madlib/eigen_backup,mjbshaw/Eigen,rotorliu/eigen,mjbshaw/Eigen,madlib/eigen_backup,mjbshaw/Eigen,rotorliu/eigen,rotorliu/eigen,mjbshaw/Eigen,madlib/eigen_backup,rotorliu/eigen,madlib/eigen_backup