Empty children arrays about unist

[wooorm]

Oeh, thanks @eush77 for bringing up this point. I think this is wrong, and it should be as follows:

 *   must have a `type` property set to a to its namespace semantic
     `string`;

 *   may have either a `value` property set to a `string` or a `children`
-    property set to an array of one or more `Unist` nodes;
+    property set to an array of zero or more `Unist` nodes;

 *   may have a `data` property set to a `JSON.stringify`able object;

...as I agree with your points 😄