[BUG] 'children' appears as '0' when not provided

When no children are present, this generates a `0` as the button text, rather than being blank:

```tsx
import { Component } from "@geajs/core";
import type { JSX } from "@geajs/core/jsx-runtime";

export type FooButtonProps = JSX.IntrinsicElements["button"];

export class FooButton extends Component {
template({ children, class: cls, ...props }: FooButtonProps) {
return (

{children}

);
}
}

// when used like:

```

Expected behavior: The button should be blank, not show `0`.

Start at the JSX children-rendering path used by the shown FooButton template and reproduce the empty-children case with . Trace why an absent children value becomes button text, then verify that the button remains blank when no children are provided.

Written by the indexing model from the issue text.