Produce `&[T]` instead of `Option<&[T]>` for list fields (#2995)

## Motivation and Context
When the returned field is a list, there is _almost_ never a reason to
know if the returned value was null or `[]` — this addresses the 99%
case.

Before:
```rust
fn blah(&self) -> Option<&[T]> { self.blah.as_deref() }
```

After:
```rust
fn blah(&self) -> &[T] { self.blah.as_deref().unwrap_or_default() }
```

**note**: no impact on servers by default, see codegen diff.

## Description
Update accessors for lists.

## Testing
- [x] codegen diff audit (no server changes)
- [x] unit tests

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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