Fix striping of empty sequence panicking with AVX2 implementation

lightmotif/src/pli/platform/avx2.rs

@@ -406,6 +406,11 @@ unsafe fn stripe_avx2<A>(
     let mut matrix = std::mem::take(striped).into_matrix();
     matrix.resize(src_stride);
 
+    // Early exit if sequence is too empty (no allocated matrix).
+    if length == 0 {
+        return;
+    }
+
     /// Get a pointer to the matrix
     let mut out = matrix[0].as_mut_ptr();
     let out_stride = matrix.stride();

lightmotif/src/seq.rs

@@ -347,6 +347,18 @@ mod test {
     use crate::abc::Dna;
     use crate::abc::Nucleotide::*;
 
+    #[test]
+    fn test_empty() {
+        let seq = EncodedSequence::<Dna>::from_str("").unwrap();
+
+        let pli = Pipeline::generic();
+        let striped = <Pipeline<_, _> as Stripe<Dna, U2>>::stripe(&pli, &seq);
+        assert_eq!(striped.matrix().rows(), 0);
+
+        let striped = seq.to_striped();
+        assert_eq!(striped.matrix().rows(), 0);
+    }
+
     #[test]
     fn test_stripe() {
         let pli = Pipeline::generic();