test(firestore): add regression test for limit(0) serialization

For #7382

Author: liu-husong

handwritten/firestore/dev/src/reference/query-util.ts

@@ -344,7 +344,7 @@ export class QueryUtil<
                   // call to `requestStream()` will backoff should the restart
                   // fail before delivering any results.
                   let newQuery: Query<AppModelType, DbModelType>;
-                  if (!this._queryOptions.limit) {
+                  if (this._queryOptions.limit === undefined) {
                     newQuery = query;
                   } else {
                     const newLimit =

handwritten/firestore/dev/src/reference/query.ts

@@ -1608,7 +1608,7 @@ export class Query<
     structuredQuery.startAt = this.toCursor(this._queryOptions.startAt);
     structuredQuery.endAt = this.toCursor(this._queryOptions.endAt);
 
-    if (this._queryOptions.limit) {
+    if (this._queryOptions.limit !== undefined) {
       structuredQuery.limit = {value: this._queryOptions.limit};
     }
 

handwritten/firestore/dev/test/query.ts

@@ -1958,9 +1958,6 @@ describe('limit() interface', () => {
     await query.get();
   });
 
-  // Regression test: This test currently fails because limit(0) is not
-  // serialized in the query proto (limit(0) is falsy in JavaScript).
-  // This test is expected to fail until the fix is applied.
   it('handles limit(0) correctly', async () => {
     const overrides: ApiOverride = {
       runQuery: request => {