UiPlugin cannot be used without rendering

## Bevy version

0.6

## What you did

```
use bevy::prelude::*;
use bevy::ui::UiPlugin;

fn main() {
App::new()
.add_plugins(MinimalPlugins)
.add_plugin(UiPlugin)
.run();
}
```

## What actually happened

```
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', C:\Users\Alice\.cargo\registry\src\github.com-1ecc6299db9ec823\bevy_ui-0.6.0\src\render\mod.rs:62:70
```
This panic is in https://github.com/bevyengine/bevy/blob/458cb7a9e93dab3127bb99ce7bf8cfc3af18851d/crates/bevy_ui/src/render/mod.rs#L61.

## Additional information

Related to #3155.

This is essential for headless integration testing of UI logic.