Allow DiagnosticsBuffer to be applied even if DiagnosticsStore is missing. (#21550)

# Objective

- Workaround #21549.

## Solution

- Avoid panicking in `DiagnosticsBuffer::apply` if the world is missing
the `DiagnosticsStore`.
- Clear the buffer if `DiagnosticsStore` is missing to prevent growing
the buffer.

## Testing

- None.

Author: andriyDev

crates/bevy_diagnostic/src/diagnostic.rs

@@ -381,7 +381,17 @@ impl SystemBuffer for DiagnosticsBuffer {
         _system_meta: &bevy_ecs::system::SystemMeta,
         world: &mut bevy_ecs::world::World,
     ) {
-        let mut diagnostics = world.resource_mut::<DiagnosticsStore>();
+        let Some(mut diagnostics) = world.get_resource_mut::<DiagnosticsStore>() else {
+            // `SystemBuffer::apply` is called even if the system never runs. If a user uses
+            // `If<Diagnostics>`, this buffer will be applied even if we are missing
+            // `DiagnosticsStore`. So be permissive to allow these cases. See
+            // https://github.com/bevyengine/bevy/issues/21549 for more.
+
+            // Clear the buffer since we have nowhere to put those metrics and we don't want them to
+            // grow without bound.
+            self.0.clear();
+            return;
+        };
         for (path, measurement) in self.0.drain() {
             if let Some(diagnostic) = diagnostics.get_mut(&path) {
                 diagnostic.add_measurement(measurement);