Fix pagination continueCursor at end of query

convex/pagination.test.ts

@@ -26,7 +26,7 @@ test("paginate", async () => {
     { author: "sarah", body: "hello2" },
   ]);
   expect(isDone).toEqual(false);
-  const { isDone: isDone2, page: page2 } = await t.query(api.pagination.list, {
+  const { continueCursor: continueCursor2, isDone: isDone2, page: page2 } = await t.query(api.pagination.list, {
     author: "sarah",
     paginationOptions: {
       cursor: continueCursor,
@@ -39,4 +39,15 @@ test("paginate", async () => {
     { author: "sarah", body: "hello5" },
   ]);
   expect(isDone2).toEqual(true);
+
+  // Querying after done should return nothing.
+  const { isDone: isDone3, page: page3 } = await t.query(api.pagination.list, {
+    author: "sarah",
+    paginationOptions: {
+      cursor: continueCursor2,
+      numItems: 4,
+    },
+  });
+  expect(page3).toMatchObject([]);
+  expect(isDone3).toEqual(true);
 });

index.ts

@@ -371,6 +371,9 @@ class DatabaseFake {
       const { value, done } = this.queryNext(queryId);
       if (done) {
         isDone = true;
+        // We have reached the end of the query. Return a cursor that indicates
+        // "end query", which we can do with any string that isn't a valid _id.
+        continueCursor = "_end_cursor";
         break;
       }
       if (isInPage) {
@@ -387,7 +390,7 @@ class DatabaseFake {
     return {
       page,
       isDone,
-      continueCursor,
+      continueCursor: continueCursor!,
     };
   }