shader_material_glsl example crashes if you add a light with shadows_enabled

## Bevy version

0.10.0

## \[Optional\] Relevant system information

cargo 1.68.0 (115f34552 2023-02-26)

### the operating system
This crashed on two systems, they are both Intel mac machines, one macos, one linux.

#### MacOS 13.2.1
```ignore
2023-03-20T14:24:58.227437Z INFO bevy_render::renderer: AdapterInfo { name: "AMD Radeon Pro 560", vendor: 0, device: 0, device_type: DiscreteGpu, driver: "", driver_info: "", backend: Metal }
2023-03-20T14:24:58.806900Z INFO bevy_diagnostic::system_information_diagnostics_plugin::internal: SystemInfo { os: "MacOS 13.2.1", kernel: "22.3.0", cpu: "Intel(R) Core(TM) i5-7500 CPU @ 3.40GHz", core_count: "4", memory: "8.0 GiB" }
```
#### Linux 20.04 Ubuntu
```ignore
2023-03-20T14:31:46.734571Z INFO bevy_render::renderer: AdapterInfo { name: "Intel(R) Iris(R) Graphics 5100 (HSW GT3)", vendor: 32 902, device: 2606, device_type: IntegratedGpu, driver: "Intel open-source Mesa driver", driver_info: "Mesa 21.2.6", backend: Vulkan }
2023-03-20T14:31:47.526969Z INFO bevy_diagnostic::system_information_diagnostics_plugin::internal: SystemInfo { os: "Linux 20.04 Ubuntu", kernel: "5.15.0-67-generic", cpu: "Intel(R) Core(TM) i5-4278U CPU @ 2.60GHz", core_count: "2", memory: "7.6 GiB" }
```

I tried testing the examples of upstream dependency
- [`wgpu`](https://github.com/gfx-rs/wgpu) for rendering problems
- conservative-raster
thread 'main' panicked at 'Adapter does not support required features for this example: CONSERVATIVE_RASTERIZA
TION', wgpu/examples/conservative-raster/../framework.rs:200:5
- everything else works fine, including the shadow example

## What you did

I copied the example exactly from: bevy/examples/shader/shader_material_glsl.rs
and added a PointLightBundle with shadows_enabled and crash.

It also does not work with DirectionalLightBundle or SpotLightBundle with the shadows_enabled on the inner light.

This is my [repository](https://github.com/woodroww/crash_this)

### Crashes:
```rust
commands.spawn(PointLightBundle {
point_light: PointLight {
shadows_enabled: true,
..default()
},
..default()
});
```
### Runs:
```rust
commands.spawn(PointLightBundle {
point_light: PointLight {
shadows_enabled: false,
..default()
},
..default()
});
```

## What went wrong

App crashes. The fragment shader is None in the specialize function (main.rs line 113).
```rust
descriptor.fragment.as_mut().unwrap().entry_point = "main".into();
```

## Additional information

I searched around issues and doc.rs for a while, but found nothing.

## Logs
### on MacOS

```ignore
2023-03-20T14:24:58.227437Z INFO bevy_render::renderer: AdapterInfo { name: "AMD Radeon Pro 560", vendor: 0, device: 0, device_type: DiscreteGpu, driver: "", driver_info: "", backend: Metal }
2023-03-20T14:24:58.697170Z INFO bevy_winit::system: Creating new window "Bevy App" (0v0)
2023-03-20T14:24:58.806900Z INFO bevy_diagnostic::system_information_diagnostics_plugin::internal: SystemInfo { os: "MacOS 13.2.1 ", kernel: "22.3.0", cpu: "Intel(R) Core(TM) i5-7500 CPU @ 3.40GHz", core_count: "4", memory: "8.0 GiB" }
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/main.rs:113:38
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'thread 'Compute Task Pool (0)main' panicked at '' panicked at 'internal error: entered unreachable code: sending into a closed channelA system has panicked so the executor cannot continue.: RecvError', ', /Users/matt /.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.10.0/src/schedule/executor/multi_threaded.rs/Users/matt/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.10.0/src/schedule/executor/multi_threaded.rs::459 194::4560
thread '' panicked at 'called `Option::unwrap()` on a `None` value', /Users/matt/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_tasks-0.10.0/src/task_pool.rs:376:49
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: RecvError', /Users/matt/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_render-0.10.0/src/pipelined_rendering.rs:136:45
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /Users/matt/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_tasks-0.10.0/src/task_pool.rs:376:49
```

### on Linux

```ignore
2023-03-20T14:31:46.647954Z INFO bevy_winit::system: Creating new window "Bevy App" (0v0)
2023-03-20T14:31:46.648298Z INFO winit::platform_impl::platform::x11::window: Guessed window scale factor: 1
MESA-INTEL: warning: Haswell Vulkan support is incomplete
2023-03-20T14:31:46.734571Z INFO bevy_render::renderer: AdapterInfo { name: "Intel(R) Iris(R) Graphics 5100 (HSW GT3)", vendor: 32 902, device: 2606, device_type: IntegratedGpu, driver: "Intel open-source Mesa driver", driver_info: "Mesa 21.2.6", backend: Vulkan }
2023-03-20T14:31:47.526969Z INFO bevy_diagnostic::system_information_diagnostics_plugin::internal: SystemInfo { os: "Linux 20.04 U buntu", kernel: "5.15.0-67-generic", cpu: "Intel(R) Core(TM) i5-4278U CPU @ 2.60GHz", core_count: "2", memory: "7.6 GiB" }
thread 'Compute Task Pool (0)' panicked at 'called `Option::unwrap()` on a `None` value', src/main.rs:113:38
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'Compute Task Pool (0)' panicked at 'internal error: entered unreachable code: sending into a closed channel', /home/matt/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.10.0/src/schedule/executor/multi_threaded.rs:459:45
thread 'Compute Task Pool (0)' panicked at 'A system has panicked so the executor cannot continue.: RecvError', /home/matt/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.10.0/src/schedule/executor/multi_threaded.rs:194:60thread '
main' panicked at 'internal error: entered unreachable code: sending into a closed channel', /home/matt/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.10.0/src/schedule/executor/multi_threaded.rs:thread '459Compute Task Pool (1):' panicked at '45internal error: entered unreachable code: sending into a closed channel', /home/matt/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.10.0/src/schedule/executor/multi_threaded.rs:459:45
thread '' panicked at 'called `Option::unwrap()` on a `None` value', /home/matt/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_tasks-0.10.0/src/task_pool.rs:376:49
thread 'main' panicked at 'internal error: entered unreachable code: sending into a closed channel', /home/matt/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.10.0/src/schedule/executor/multi_threaded.rs:459:45
thread 'Compute Task Pool (0)' panicked at 'internal error: entered unreachable code: sending into a closed channel', /home/matt/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.10.0/src/schedule/executor/multi_threaded.rs:459:45
thread 'Compute Task Pool (0)' panicked at 'called `Result::unwrap()` on an `Err` value: RecvError', /home/matt/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_render-0.10.0/src/pipelined_rendering.rs:136:45
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /home/matt/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_tasks-0.10.0/src/task_pool.rs:376:49
```