Fix issue with scoring of an empty range of rows

lightmotif/src/dense.rs

lightmotif/src/pli/mod.rs

lightmotif/src/pli/platform/avx2.rs

lightmotif/src/pli/platform/neon.rs

lightmotif/src/pli/platform/sse2.rs

lightmotif/src/scan.rs

lightmotif/src/seq.rs