Allow empty string text nodes

Currently, if you try to render a node like this:
```js
text('')
```
It will go through this part of the code to get HTML escaped, however, because the `||` operator considers `''` a falsy value, it goes for `node.name` instead, which does not exist on a VDOM node. The first line in this `escapeHtml` function converts this `undefined` value to string, which ends up being the string `'undefined'`.

I think this is a bug?

Hope this helps!

src/index.js

@@ -27,6 +27,7 @@ const voidElements = new Set([
 
 // credits to https://github.com/component/escape-html
 export function escapeHtml(value) {
+  if (value == null) return '';
   const str = '' + value
   if (typeof value === 'number') {
     // better performance for safe values

test/index.test.js

@@ -478,6 +478,28 @@ describe('renderToString(view, state, actions)', () => {
     expect(html).toBe('<div>foo bar baz</div>')
   })
 
+  it('should correctly render empty text nodes', () => {
+    const VNode = {
+      tag: 'div',
+      props: {},
+      key: null,
+      children: [
+        {
+          tag: '',
+          props: {},
+          key: null,
+          children: [],
+          type: 3,
+          node: null,
+        },
+      ],
+      type: 1,
+      node: null,
+    }
+    const html = renderToString(VNode)
+    expect(html).toBe('<div></div>')
+  })
+
   it('should support Hyperapp V2 lazy nodes', () => {
     const VNode = {
       lazy: {