Accept undefined/null in `append()`

[zefir-git]

Accept undefined/null in append() so that conditional appending is possible, without leaving the component method chain. Example:

// Current beheaviour
private method(foo: Component, bar: Component, baz?: Component) {
    const div = new Component("div")
        .append(foo);
    if (baz) div.append(baz);
    div.append(bar);
    return div;
}

// Proposed new behaviour
private method(foo: Component, bar: Component, baz?: Component) {
    return new Component("div")
        .append(foo)
        .append(baz)
        .append(bar);
}

The idea is to enable the use of:

.append(boolean ? Component : null)