fix(browser): clear idle blink timer when handle is 0

_clearIdleTimer() used truthiness on _idleTimeout, so a setTimeout
handle of 0 would not be cleared. Compare against undefined, consistent
with RenderDebouncer rAF handling.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/browser/renderer/dom/DomRenderer.ts

@@ -694,7 +694,7 @@ class CursorBlinkStateManager {
   }
 
   private _clearIdleTimer(): void {
-    if (this._idleTimeout) {
+    if (this._idleTimeout !== undefined) {
       this._coreBrowserService.window.clearTimeout(this._idleTimeout);
       this._idleTimeout = undefined;
     }