Return empty array instead of undefined in mapTimestampToAxisTicksByFormat

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/module/timeLabel.js

@@ -251,4 +251,5 @@ export const mapTimestampToAxisTicksByFormat = (timeStampArr, timeFormat) => {
   if (timeStampArr) {
     return timeStampArr.map((ts) => moment(ts).format(timeFormat));
   }
+  return [];
 };

tests/unit/module/timeLabel.test.js

@@ -168,7 +168,7 @@ describe("mapTimestampToAxisTicksByFormat", () => {
     expect(result[0]).toBe("2023/01/15");
   });
 
-  it("returns undefined for falsy input", () => {
-    expect(mapTimestampToAxisTicksByFormat(null, "YYYY")).toBeUndefined();
+  it("returns empty array for falsy input", () => {
+    expect(mapTimestampToAxisTicksByFormat(null, "YYYY")).toEqual([]);
   });
 });