fix(studio): guard against null tag in timeline track style

getTrackStyle() can receive a falsy tag at runtime (e.g. empty string
from timeline element defaults), causing toLowerCase() and startsWith()
to throw. Default to "div" when tag is falsy.

Author: miguel-heygen

packages/studio/src/player/components/timelineIcons.tsx

@@ -39,8 +39,9 @@ const ICONS: Record<string, ReactNode> = {
 };
 
 export function getTrackStyle(tag: string): TrackVisualStyle {
-  const trackStyle = getTimelineTrackStyle(tag);
-  const normalized = tag.toLowerCase();
+  const safeTag = tag || "div";
+  const trackStyle = getTimelineTrackStyle(safeTag);
+  const normalized = safeTag.toLowerCase();
   const icon =
     normalized.startsWith("h") && normalized.length === 2 && "123456".includes(normalized[1] ?? "")
       ? ICONS.h1