`Deferred::apply` runs even if the system never ran.

## Bevy version and features

- Main @ fcd5c265

## What you did

```rust
// My app does not include the diagnostics plugin, so the DiagnosticsBuffer within Diagnostics will fail to apply.

fn my_optional_diagnostics_system(mut diagnostics: If) {
// I only want these measurements to be sent when diagnostics exist.
}
```

## What went wrong

The SystemBuffer in DiagnosticsBuffer gets applied, resulting in a panic since the DiagnosticsStore does not exist.

While the DiagnosticsBuffer probably should avoid panicking in this case, we also probably shouldn't be applying `Deferred` if the system never ran! This is most visible using the `If` type to automatically skip systems when the DiagnosticsStore is missing.

Start by reproducing the optional diagnostics example and trace Deferred::apply when the If-wrapped system is skipped. Inspect the interaction between If, DiagnosticsBuffer, and the scheduler; done means skipped systems no longer apply deferred state, while systems that run still apply it without causing the reported panic.

Written by the indexing model from the issue text.