JSX children should be considered `JsxNode | NestedArray<JsxNode>` instead of always `NestedArray<JsxNode>`.

[thejimmylin]

Because a component has this type:

type Component = (props: JsxProps) => Renderable;
type Renderable = JsxNode | NestedArray<JsxNode>;

If we set children as children[0] when children.length is 0, the type of JSX children would be the same as the return type of a component.