Simplify MountRenderer.unmount

Using current versions of Preact, `render(null, container)` is the idiomatic way
to unmount a component.

src/MountRenderer.ts

@@ -4,7 +4,7 @@ import type {
   RSTNode,
 } from 'enzyme';
 import type { VNode } from 'preact';
-import { h, createElement } from 'preact';
+import { createElement } from 'preact';
 import { act } from 'preact/test-utils';
 import type { PreactAdapterOptions } from './Adapter.js';
 
@@ -74,10 +74,7 @@ export default class MountRenderer implements AbstractMountRenderer {
   }
 
   unmount() {
-    // A custom tag name is used here to work around
-    // https://github.com/developit/preact/issues/1288.
-    render(h('unmount-me', {}), this._container);
-    this._container.innerHTML = '';
+    render(null, this._container);
   }
 
   getNode() {

src/preact10-rst.ts

@@ -182,7 +182,7 @@ function rstNodeFromComponent(vnode: VNode, component: Component): RSTNode {
 /**
  * Convert the Preact components rendered into `container` into an RST node.
  */
-export function getNode(container: HTMLElement): RSTNode {
+export function getNode(container: HTMLElement): RSTNode | null {
   const rendered = getLastVNodeRenderedIntoContainer(container);
   const rstNode = rstNodeFromVNode(rendered);
 
@@ -193,6 +193,8 @@ export function getNode(container: HTMLElement): RSTNode {
   if (Array.isArray(rstNode)) {
     if (rstNode.length === 1) {
       return rstNode[0] as RSTNode;
+    } else if (rstNode.length === 0) {
+      return null;
     } else {
       throw new Error(
         'Root element must not be a fragment with multiple children'