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)