Register unit type alongside the other primitives (#21298)

# Objective

Primitive types are automatically registered when a `TypeRegistry` is
created, except for unit type `()`.

## Solution

Register `()` alongside other primitives.

## Testing

Run tests in `bevy_reflect`, tested with my code that `()` is being
registered.

Author: MichalGniadek

crates/bevy_reflect/src/type_registry.rs

@@ -100,6 +100,7 @@ impl TypeRegistry {
     /// Create a type registry with default registrations for primitive types.
     pub fn new() -> Self {
         let mut registry = Self::empty();
+        registry.register::<()>();
         registry.register::<bool>();
         registry.register::<char>();
         registry.register::<u8>();