Fix issue with empty si element in xlsx shared string element

src/xlsx/cells_reader.rs

@@ -353,9 +353,14 @@ fn read_v<'s>(
     };
     match get_attribute(c_element.attributes(), QName(b"t"))? {
         Some(b"s") => {
-            // shared string
+            // Cell value is an index into the shared string table.
             let idx = atoi_simd::parse::<usize>(v.as_bytes()).unwrap_or(0);
-            Ok(DataRef::SharedString(&strings[idx]))
+            match strings.get(idx) {
+                Some(shared_string) => Ok(DataRef::SharedString(shared_string)),
+                None => Err(XlsxError::Unexpected(
+                    "Cell string index not found in shared strings table",
+                )),
+            }
         }
         Some(b"b") => {
             // boolean

src/xlsx/mod.rs

@@ -1807,6 +1807,12 @@ where
                 is_phonetic_text = true;
             }
             Ok(Event::End(ref e)) if e.name() == closing => {
+                if rich_buffer.is_none() {
+                    // An empty <s></s> element, without <t> or other
+                    // subelements, is treated as a valid empty string in Excel.
+                    rich_buffer = Some(String::new());
+                }
+
                 return Ok(rich_buffer);
             }
             Ok(Event::End(ref e)) if e.local_name().as_ref() == b"rPh" => {

tests/test.rs

@@ -2264,3 +2264,16 @@ fn test_xlsb_case_insensitive_part_name() {
 fn test_xlsx_backward_slash_part_name() {
     let _: Xlsx<_> = wb("issue_530.xlsx");
 }
+
+// Test for issue #573 where a shared string doesn't have a <t> sub-element.
+// This is unusual but valid according to the xlsx specification.
+#[test]
+fn test_xlsx_empty_shared_string() {
+    let mut excel: Xlsx<_> = wb("empty_shared_string.xlsx");
+    let range = excel.worksheet_range("Sheet1").unwrap();
+
+    range_eq!(
+        range,
+        [[String("abc".to_string())], [String("".to_string())]]
+    );
+}