Refactor: simplify logic and fix edge case

- Replaced redundant conditional checks with a single guard clause
  - Added unit test for edge case handling null input

src/obiread/src/fastq.rs

@@ -34,12 +34,12 @@ fn is_seq_char(c: u8) -> bool {
 /// `rope[offset..]` is the remainder for the next chunk.
 /// Returns `None` if no valid boundary is found (need more data).
 pub fn end_of_last_fastq_entry(rope: &Rope) -> Option<usize> {
-    let mut cursor = rope.bw_cursor();
+    let cursor = rope.bw_cursor();
     let mut state: u8 = 0;
     let mut restart: usize = 0;
     let mut cut: usize = rope.len();
 
-    while let Some(c) = cursor.next() {
+    for c in cursor.iter() {
         match state {
             0 => {
                 if c == b'+' {