Fix Keyboard observer not updating SliderValue (#19661)

# Objective

When the `CoreSlider`s `on_change` is set to None, Keyboard input, like
ArrowKeys, does not update the `SliderValue`.

## Solution

Handle the missing case, like it is done for Pointer.

## Testing

- Did you test these changes?
Yes: core_widgets & core_widgets_observers
in both examples one has to remove / comment out the setting of
`CoreSlider::on_change` to test the case of `on_change` being none.

- Are there any parts that need more testing?
No not that I am aware of.

- How can other people (reviewers) test your changes? Is there anything
specific they need to know?
Yes: core_widgets & core_widgets_observers
in both examples one has to remove / comment out the setting of
`CoreSlider::on_change` to test the case of `on_change` being none.

- If relevant, what platforms did you test these changes on, and are
there any important ones you can't test?
I tested on linux + wayland. But it is unlikely that it would effect
outcomes.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_core_widgets/src/core_slider.rs

@@ -394,6 +394,10 @@ fn slider_on_key_input(
             trigger.propagate(false);
             if let Some(on_change) = slider.on_change {
                 commands.run_system_with(on_change, new_value);
+            } else {
+                commands
+                    .entity(trigger.target())
+                    .insert(SliderValue(new_value));
             }
         }
     }