Fix support for void nodes

Closes GH-5.
Closes GH-6.

Reviewed-by: Titus Wormer <[email protected]>

Author: staltz

index.js

@@ -25,7 +25,7 @@ export function toString(node, separator = '') {
   if (typeof node.value === 'string') return node.value
 
   // @ts-ignore Looks like a list of nodes or parent.
-  children = 'length' in node ? node : node.children
+  children = ('length' in node ? node : node.children) || []
 
   // Shortcut: This is pretty common, and a small performance win.
   if (children.length === 1 && 'value' in children[0]) {

test.js

@@ -50,5 +50,7 @@ test('toString()', function (t) {
     'should support separators'
   )
 
+  t.equal(toString(u('foo')), '', 'should support voids')
+
   t.end()
 })