Normalize zero-size data out pointers on the create side (createArrayBuffer/createBuffer/createBufferCopy)

## Context

Noted while reviewing #56. That PR normalizes the *get* side (`napi_get_arraybuffer_info` etc.) so a zero-length result with a `NULL`/arbitrary data pointer can no longer produce a Zig slice that violates the non-null pointer invariant.

The *create* side has the same class of issue: `Env.createArrayBuffer`, `Env.createBuffer`, and `Env.createBufferCopy` take `out: ?*[*]u8`, and `TypedArray.from`/`alloc` pass a non-nullable `buf_ptr` into them. For `size == 0`, Node-API may write `NULL` into that non-nullable `[*]u8`, after which `from` does `@ptrCast(@alignCast(buf_ptr))`.

`float64Scale(new Float64Array(0), 2.5)` (added in #56's tests) exercises this path and currently passes — but only because address 0 happens to satisfy every alignment check. It is the same type-invariant violation, accidentally safe rather than correct.

## Proposal

Apply the same nullable-pointer normalization as #56: receive the out pointer as `?*anyopaque` (or make the out param nullable-aware), and have `TypedArray.from`/`alloc` short-circuit to an empty/zero-length result for `len == 0` before any alignment cast.

Refs: `src/Env.zig` (createArrayBuffer/createBuffer/createBufferCopy), `src/js/typed_arrays.zig` (`from`, `alloc`)