Correctly handle UI hierarchy without a camera (#12816)

# Objective
Add support so bevy_ui can correctly handle an UI hierarchy without a
camera present.

- Fixes #12184

## Solution

As there was no default behavior for what should happen when a camera is
not present in a UI hierarchy, the solution
was based in defining that default behavior and improving the overall
handling of this "exception".


## Changelog

- Create default values to be used in upsert_node
- Add flag to control warnings about no camera present
- Create unit test no_camera_ui (to test if ui handles no camera
present)

crates/bevy_ui/src/layout/mod.rs

@@ -32,6 +32,12 @@ pub struct LayoutContext {
 }
 
 impl LayoutContext {
+    pub const DEFAULT: Self = Self {
+        scale_factor: 1.0,
+        physical_size: Vec2::ZERO,
+        min_size: 0.0,
+        max_size: 0.0,
+    };
     /// create new a [`LayoutContext`] from the window's physical size and scale factor
     fn new(scale_factor: f32, physical_size: Vec2) -> Self {
         Self {
@@ -43,6 +49,12 @@ impl LayoutContext {
     }
 }
 
+impl Default for LayoutContext {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 #[derive(Debug, Error)]
 pub enum LayoutError {
     #[error("Invalid hierarchy")]
@@ -156,6 +168,8 @@ pub fn ui_layout_system(
                 );
                 ui_surface.upsert_node(entity, &style, &layout_context);
             }
+        } else {
+            ui_surface.upsert_node(entity, &Style::default(), &LayoutContext::default());
         }
     }
     scale_factor_events.clear();
@@ -1000,4 +1014,65 @@ mod tests {
             }
         }
     }
+
+    #[test]
+    fn no_camera_ui() {
+        let mut world = World::new();
+        world.init_resource::<UiScale>();
+        world.init_resource::<UiSurface>();
+        world.init_resource::<Events<WindowScaleFactorChanged>>();
+        world.init_resource::<Events<WindowResized>>();
+        // Required for the camera system
+        world.init_resource::<Events<WindowCreated>>();
+        world.init_resource::<Events<AssetEvent<Image>>>();
+        world.init_resource::<Assets<Image>>();
+        world.init_resource::<ManualTextureViews>();
+
+        // spawn a dummy primary window and camera
+        world.spawn((
+            Window {
+                resolution: WindowResolution::new(WINDOW_WIDTH, WINDOW_HEIGHT),
+                ..default()
+            },
+            PrimaryWindow,
+        ));
+
+        let mut ui_schedule = Schedule::default();
+        ui_schedule.add_systems(
+            (
+                // UI is driven by calculated camera target info, so we need to run the camera system first
+                bevy_render::camera::camera_system::<OrthographicProjection>,
+                update_target_camera_system,
+                apply_deferred,
+                ui_layout_system,
+            )
+                .chain(),
+        );
+
+        let ui_root = world
+            .spawn(NodeBundle {
+                style: Style {
+                    width: Val::Percent(100.),
+                    height: Val::Percent(100.),
+                    ..default()
+                },
+                ..default()
+            })
+            .id();
+
+        let ui_child = world
+            .spawn(NodeBundle {
+                style: Style {
+                    width: Val::Percent(100.),
+                    height: Val::Percent(100.),
+                    ..default()
+                },
+                ..default()
+            })
+            .id();
+
+        world.entity_mut(ui_root).add_child(ui_child);
+
+        ui_schedule.run(&mut world);
+    }
 }