fix: children.map when new child return null

[ishenli]

ref: #419

When the new child is null, should ignore , like React

[paranoidjk]

why not return children.map(fn).filter(item => item !== null);

[ishenli]

when the children has true or false, React will ignore them, see the demo

// demo
let instance = (
    <div>
      {'123'}
      {true}
      {false}
    </div>
);
class Hello extends React.Component {
  render() {
    console.info(instance.props.children); //  ["123", true, false]
    const ChildrenMap = React.Children.map(instance.props.children, item => item);
    console.info(ChildrenMap); // ["123"]
    return <div>
     {ChildrenMap}
    </div>
  }
}
ReactDOM.render(<Hello/>, document.getElementById('test'));    

[paranoidjk]

ok, i see we need to skip boolean and null, undefined

[developit]

Doesn't it only skip rendering?

[ishenli]

Yeah, react does much more in React.Children.Map, but I think skip rendering is the the main difference between react-compat and react. The main purpose of this pull request is to improve compatibility.

[developit]

I'm good merging this, but I'd like the loop converted to a for loop for size reasons. I can do it after merge though!

[likezero]

Hi，@developit can you merge this PR?

[developit]

I'm still confused how it's possible to end up with props.children containing values like null or undefined. Preact strips those out - see this demo:
https://jsfiddle.net/developit/z97ua3m3/

So it would seem that, regardless of whether this PR is merged or not, Preact will still have removed the empty values prior to map() running, right?

Setting that aside, I think this could be simplified a bit? Something like:

	map(children, fn, ctx) {
		if (children == null) return null;
		children = Children.toArray(children);
		if (ctx) fn = fn.bind(ctx);
		return children.filter(Boolean).map(fn).filter(Boolean);
	}

[maxsbelt]

@developit children property can contain null values. I created simple example for preact@8.3.1: https://codesandbox.io/s/wwnx5q1277?expanddevtools=1