docs: change ActivatedRouteStub code sample to work with strictNullChecks on (#41559)

convertToParamMap() does not accept undefined.
Fix code sample so it can be used in strict mode without requiring changes.

PR Close #41559

aio/content/examples/testing/src/testing/activated-route-stub.ts

@@ -21,8 +21,8 @@ export class ActivatedRouteStub {
   /** The mock paramMap observable */
   readonly paramMap = this.subject.asObservable();
 
-  /** Set the paramMap observables's next value */
-  setParamMap(params?: Params) {
+  /** Set the paramMap observable's next value */
+  setParamMap(params: Params = {}) {
     this.subject.next(convertToParamMap(params));
   }
 }