DirectionalLight shadow gets eaten, when another without shadow_enabled is present

## Bevy version

0.16.0-rc.1+

## What you did

- spawn a `DirectionalLight` with `shadow_enabled: true`
- spawn a 2nd `DirectionalLight` with `shadow_enabled: false` (even with `illuminance: 0.`)

## What went wrong

Not sure if intended, but the shadow is eaten, in 0.15 it mixed them. 🦆

Reproduce the issue in Bevy 0.16.0-rc.1+ using two DirectionalLights, one with shadow_enabled enabled and one disabled, then compare the result with Bevy 0.15. Trace the directional-light shadow and lighting behavior in the rendering subsystem; done means the enabled light's shadow is preserved when the second light has shadows disabled, including when its illuminance is zero.

Written by the indexing model from the issue text.