Make asset handles `!Default`

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

[Handle](https://github.com/bevyengine/bevy/blob/b13472dae495ecc7d1a80cbe1ad12718c0de9262/crates/bevy_asset/src/handle.rs#L61) currently requires `Default` to be implemented, which reduces type safety because for many `Assets` no default makes sense. Removing this requirement would also partially solve #1201, because when creating `Text` and filling out the values with `..Default::default()` `style.font` will be set to an invalid font Handle and fail to render.

## What solution would you like?

The removal of the requirement that all `Handles` have to implement `Default`. The reason this requirement exists is because `Handle` derives `Reflect`.

## What alternative(s) have you considered?

Runtime errors for default handle values where `Default` is invalid, but this delays the problem feedback and increases iteration time compared to getting the error at compile time.

Start with crates/bevy_asset/src/handle.rs at the linked Handle definition and inspect why it derives Reflect and requires Default. Trace the affected Handle and Assets usages, then verify that removing the Default requirement preserves reflection while preventing invalid default handles from compiling.

Written by the indexing model from the issue text.