fix: render sidebar navigation item children when parent has no URL

[truffle-dev]

Description

Closes #19989.

When a top-level `NavigationItem` is registered via `->navigationItems([...])` with `->childItems([...])` and no `->url(...)` on the parent, the children never render in the sidebar. The reporter's repro (also in https://github.com/gigili/filament-navitem-bug):

```php
->navigationItems([
NavigationItem::make('tools')
->label(__('landing.nav.tools'))
->icon(Heroicon::WrenchScrewdriver)
->childItems([
NavigationItem::make('all')->label('test'),
]),
])
```

The gate at `packages/panels/resources/views/components/sidebar/item.blade.php:115` requires the parent or one of its children to be `active` before the children-`

` is emitted:

```blade
@if (($active || $activeChildItems) && $childItems)
```

For a parent with no URL, `$active` is always false (no URL to match against the current route) and `$activeChildItems` is false (the children have no URLs either). So the gate fails and the children-`

` is never written to the DOM.

That condition is intentional for sub-navigation rendered inside a page: showing the route-tree only when on the matching route is the documented behavior. It is wrong for the top-level case where the parent acts as a section header rather than a navigation destination.

This change adds `blank($url)` to the OR chain so the children render whenever the parent has no URL:

```blade
@if ($childItems && (blank($url) || $active || $activeChildItems))
```

When the parent has a URL, the existing gate fires unchanged; sub-navigation behavior is preserved. When the parent has no URL, the children render unconditionally so the user can reach them.

Visual changes

Before: a top-level `NavigationItem` with no URL and `childItems` renders only the parent button. Clicking the button does nothing and the children are absent from the DOM.

After: the children render in a `<ul class="fi-sidebar-sub-group-items">` below the parent, matching how children render today when the parent or a child is active.

Functional changes

• Code style has been fixed by running the `composer cs` command.
• Changes have been tested to not break existing functionality.
• Documentation is up-to-date.

[GeminiDev1]

Should a parent item with a ->url(...) still render its childItems?

Or is that intentional to hide childItems when parent URL is set?

[truffle-dev]

URL'd parents are intentionally unchanged. The new clause is $childItems && (blank($url) || $active || $activeChildItems): blank($url) only opens the gate when no URL is set; with a URL, the original $active || $activeChildItems still gates the children-<ul>, so URL'd parents continue to show their children only on the matching route (the documented sub-navigation pattern). The added path is purely the top-level case where a parent acts as a section header rather than a destination.