Engine: a JSON workbook with no sheets keeps its macros

The JSON reader replaces a workbook with an empty sheet list by a new
one before macros::load reads the file's macro entries, so they were
lost with no warning. The entries are carried across the reset. The
reset itself, which also starts the names and styles afresh, is
unchanged.

crates/engine/src/macros/tests.rs

@@ -514,3 +514,16 @@ fn naming_clashing_macros_takes_linear_work() {
     taken.insert("a (2)");
     assert_eq!(taken.unique("a"), "a (4)");
 }
+
+#[test]
+fn a_json_workbook_with_no_sheets_keeps_its_macros() {
+    // A JSON file with an empty sheet list opens as a new workbook; its macros come along.
+    let mut wb = Workbook::new();
+    wb.macros.push(sample_macro("A"));
+    let mut v: serde_json::Value = serde_json::from_slice(&crate::io::save_bytes(&wb, "book.json").unwrap()).unwrap();
+    v["sheets"] = json!([]);
+    let (back, warnings) = crate::io::open_bytes("book.json", v.to_string().as_bytes()).unwrap();
+    assert!(warnings.is_empty(), "{warnings:?}");
+    assert_eq!(back.sheets.len(), 1);
+    assert_eq!(back.macros, wb.macros);
+}