Return empty array instead of undefined in mapTimestampToAxisTicksByFormat

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

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([]);
   });
 });