Ignore Default requirements when using unit structs in required components

## What problem does this solve or what need does it fill?

In order to add a unit struct to a components required components you need to implement Default on it, which is unneeded as unit structs only have one state.

## What solution would you like?

Ignore the Default requirements for unit structs

```rust
#[derive(Component)]
pub struct Player;

#[derive(Component)]
#[require(Player)]
pub struct Character {
health: u32,
}
```

## What alternative(s) have you considered?

Deriving Default on all unit components which is a bit tedious.

Start by locating the Rust implementation of the Component derive and the #[require] handling, then inspect how required components currently obtain Default. Use the unit-struct example as the acceptance case: a unit component should work without deriving Default, while the Character example should still compile and initialize its required component.

Written by the indexing model from the issue text.