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

[andriyDev]

Bevy version and features

• Main @ fcd5c26

What you did

// 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<Diagnostics>) {
  // 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.