Fix type issue by setting the attribute separately

Co-authored-by: ws-gregm <[email protected]>

Author: gfrancz

Diff for: packages/grpc-js-xds/src/xds-client.ts

@@ -690,7 +690,8 @@ class LrsCallState {
 
   destroy() {
     if (this.statsTimer) {
-      this.statsTimer = clearInterval(this.statsTimer);
+      clearInterval(this.statsTimer);
+      this.statsTimer = null;
     }
     return null;
   }