add regression tests and fix bug dropping first value

src/cursor/abstract_cursor.ts

@@ -742,7 +742,7 @@ export function next<T>(
     // All cursors must operate within a session, one must be made implicitly if not explicitly provided
     cursor[kInit]((err, value) => {
       if (err) return callback(err);
-      if (value) {
+      if (value != null) {
         return callback(undefined, value);
       }
       return next(cursor, blocking, callback);

test/integration/node-specific/abstract_cursor.test.ts

@@ -0,0 +1,45 @@
+import { expect } from 'chai';
+
+import { Collection, MongoClient } from '../../../src';
+
+describe('class AbstractCursor', function () {
+  let client: MongoClient;
+
+  let collection: Collection;
+  beforeEach(async function () {
+    client = await this.configuration.newClient().connect();
+
+    collection = client.db('abstract_cursor_integration').collection('test');
+
+    await collection.insertMany(Array.from({ length: 5 }, (_, index) => ({ index })));
+  });
+
+  afterEach(async function () {
+    await collection.deleteMany({});
+    await client.close();
+  });
+
+  context('toArray() with custom transforms', function () {
+    const falseyValues = [0, NaN, '', false];
+    for (const value of falseyValues) {
+      it(`supports mapping to falsey value '${value}'`, async function () {
+        const cursor = collection.find();
+        cursor.map(() => value);
+
+        const result = await cursor.toArray();
+
+        const expected = Array.from({ length: 5 }, () => value);
+        expect(result).to.deep.equal(expected);
+      });
+    }
+
+    it('does not support mapping to `null`', async function () {
+      const cursor = collection.find();
+      cursor.map(() => null);
+
+      const result = await cursor.toArray();
+
+      expect(result).to.deep.equal([]);
+    });
+  });
+});