fix: don't fail exportData calls on empty datasets (apify#3115)

CSV exports on empty datasets caused errors because of reading the first item for the CSV header.

Additionally, the changes also include the fixes from apify#2734 for `BasicCrawler.exportData`.

---------

Co-authored-by: Jindřich Bär <jindrichbar@gmail.com>

Author: Mirza-Samad-Ahmed-Baig

packages/basic-crawler/src/internals/basic-crawler.ts

@@ -1240,7 +1240,20 @@ export class BasicCrawler<Context extends CrawlingContext = BasicCrawlingContext
         const items = await dataset.export(options);
 
         if (format === 'csv') {
-            const value = stringify([Object.keys(items[0]), ...items.map((item) => Object.values(item))]);
+            let value: string;
+            if (items.length === 0) {
+                value = '';
+            } else {
+                const keys = Object.keys(items[0]);
+
+                value = stringify([
+                    keys,
+                    ...items.map((item) => {
+                        return keys.map((k) => item[k]);
+                    }),
+                ]);
+            }
+
             await ensureDir(dirname(path));
             await writeFile(path, value);
             this.log.info(`Export to ${path} finished!`);

test/core/crawlers/basic_crawler.test.ts

@@ -1716,6 +1716,21 @@ describe('BasicCrawler', () => {
             await rm(`${tmpDir}/result.json`);
         });
 
+        test('exports do not fail on empty dataset', async () => {
+            const crawler = new BasicCrawler();
+
+            await crawler.exportData(`${tmpDir}/result.csv`);
+            await crawler.exportData(`${tmpDir}/result.json`);
+
+            const csv = await readFile(`${tmpDir}/result.csv`);
+            expect(csv.toString()).toBe('');
+            const json = await readFile(`${tmpDir}/result.json`);
+            expect(json.toString()).toBe('[]\n');
+
+            await rm(`${tmpDir}/result.csv`);
+            await rm(`${tmpDir}/result.json`);
+        });
+
         test('should expose pushData helper', async () => {
             const crawler = new BasicCrawler({
                 requestHandler: async ({ pushData }) => pushData(payload),