fix(types): improve and test bundleRenderer.renderToString Promise types

packages/vue-server-renderer/types/index.d.ts

@@ -18,6 +18,7 @@ interface Renderer {
 interface BundleRenderer {
   renderToString(callback: RenderCallback): void;
   renderToString(context: object, callback: RenderCallback): void;
+  renderToString(): Promise<string>;
   renderToString(context: object): Promise<string>;
 
   renderToStream(context?: object): Readable;

types/test/ssr-test.ts

@@ -95,6 +95,14 @@ bundleRenderer.renderToString(context, (err, html) => {
   const res: string = html;
 });
 
+bundleRenderer.renderToString().then(html => {
+  const res: string = html;
+});
+
+bundleRenderer.renderToString(context).then(html => {
+  const res: string = html;
+});
+
 bundleRenderer.renderToStream(context).on('data', chunk => {
   const html = chunk.toString();
 });