fix graphical bug when empty span not aligned to char boundary

Previous output looked like this:

---- single_line_with_wide_char_unaligned_span_empty stdout ----
Error: oops::my::bad

  × oops!
   ╭─[bad_file.rs:2:4]
 1 │ source
 2 │   👼🏼text
   ·   ─▲
   ·    ╰── this bit here
 3 │     here
   ╰────
  help: try doing it better next time?

Note that the .max(start + 1) term is still necessary in the nonempty
branch, since it's possible to have a nonempty span covering zero-width
text.

src/handlers/graphical.rs

@@ -718,7 +718,11 @@ impl GraphicalReportHandler {
                 let byte_start = hl.offset();
                 let byte_end = hl.offset() + hl.len();
                 let start = self.visual_offset(line, byte_start, true).max(highest);
-                let end = self.visual_offset(line, byte_end, false).max(start + 1);
+                let end = if hl.len() == 0 {
+                    start + 1
+                } else {
+                    self.visual_offset(line, byte_end, false).max(start + 1)
+                };
 
                 let vbar_offset = (start + end) / 2;
                 let num_left = vbar_offset - start;

tests/graphical.rs

@@ -1321,3 +1321,40 @@ fn single_line_with_wide_char_unaligned_span_end() -> Result<(), MietteError> {
     assert_eq!(expected, out);
     Ok(())
 }
+
+#[test]
+fn single_line_with_wide_char_unaligned_span_empty() -> Result<(), MietteError> {
+    #[derive(Debug, Diagnostic, Error)]
+    #[error("oops!")]
+    #[diagnostic(code(oops::my::bad), help("try doing it better next time?"))]
+    struct MyBad {
+        #[source_code]
+        src: NamedSource,
+        #[label("this bit here")]
+        highlight: SourceSpan,
+    }
+
+    let src = "source\n  👼🏼text\n    here".to_string();
+    let err = MyBad {
+        src: NamedSource::new("bad_file.rs", src),
+        highlight: (10, 0).into(),
+    };
+    let out = fmt_report(err.into());
+    println!("Error: {}", out);
+    let expected = r#"oops::my::bad
+
+  × oops!
+   ╭─[bad_file.rs:2:4]
+ 1 │ source
+ 2 │   👼🏼text
+   ·   ▲
+   ·   ╰── this bit here
+ 3 │     here
+   ╰────
+  help: try doing it better next time?
+"#
+    .trim_start()
+    .to_string();
+    assert_eq!(expected, out);
+    Ok(())
+}