Fix is_grapheme_boundary panic on empty rope (#24)

`unwrap()` at `src/rope/utils.rs:172`:

https://github.com/nomad/crop/blob/ae8ac1669e1024df42b18ec3e26c3f8d933cebc2/src/rope/utils.rs#L172

If the rope is empty, the `chunks` iterator won't yield any items. We
can avoid hitting this case, and do less work, if we recognize that an
empty rope has one valid grapheme boundary, at byte offset 0.

---------

Co-authored-by: Riccardo Mazzarini <[email protected]>

Author: evanrelf

src/rope/utils.rs

@@ -158,6 +158,10 @@ pub(super) fn is_grapheme_boundary(
 
     debug_assert!(byte_offset <= byte_len);
 
+    if byte_len == 0 {
+        return true;
+    }
+
     let mut cursor = GraphemeCursor::new(0, byte_len, true);
     cursor.set_cursor(byte_offset);
 

tests/graphemes.rs

@@ -138,3 +138,9 @@ fn graphemes_is_boundary_out_of_bounds() {
     let r = Rope::from("🇷🇸🇮🇴");
     assert!(r.is_grapheme_boundary(17));
 }
+
+#[cfg(feature = "graphemes")]
+#[test]
+fn graphemes_is_boundary_empty_rope() {
+    assert!(Rope::new().is_grapheme_boundary(0));
+}