fix(core): ensure initial value of QueryList length (#21980) (#21982)

Set initial value of `length` to `0`.

Fixes regression introduced by e544742156 (diff-a85dbe0991a7577ea24b49374e9ae90b) where the `length` property ceased to have initial value.

Closes #21980

PR Close #21982

packages/core/src/linker/query_list.ts

@@ -41,7 +41,7 @@ export class QueryList<T>/* implements Iterable<T> */ {
   private _results: Array<T> = [];
   public readonly changes: Observable<any> = new EventEmitter();
 
-  readonly length: number;
+  readonly length: number = 0;
   readonly first: T;
   readonly last: T;
 

packages/core/test/linker/query_list_spec.ts

@@ -23,6 +23,22 @@ import {getDOM} from '@angular/platform-browser/src/dom/dom_adapter';
 
     function logAppend(item: any /** TODO #9100 */) { log += (log.length == 0 ? '' : ', ') + item; }
 
+    describe('dirty and reset', () => {
+
+      it('should initially be dirty and empty', () => {
+        expect(queryList.dirty).toBeTruthy();
+        expect(queryList.length).toBe(0);
+      });
+
+      it('should be not dirty after reset', () => {
+        expect(queryList.dirty).toBeTruthy();
+        queryList.reset(['one', 'two']);
+        expect(queryList.dirty).toBeFalsy();
+        expect(queryList.length).toBe(2);
+      });
+
+    });
+
     it('should support resetting and iterating over the new objects', () => {
       queryList.reset(['one']);
       queryList.reset(['two']);