fix(napi): normalize empty view data pointers

[GrapeBaBa]

Motivation

Node-API permits ArrayBuffer, Buffer, TypedArray, and DataView data out parameters to be NULL or an arbitrary pointer when the reported length is zero. This commonly occurs for empty and detached values. The previous binding received those results into non-null [*]u8 storage, so a valid Node-API result could create a zero-length Zig slice whose pointer violates the slice type invariant. Normalizing this case at the binding boundary keeps the public slice API safe, while the TypedArray zero-length branch avoids applying element-alignment casts to an empty byte-slice sentinel.

Summary

• represent Node-API void* data out parameters as nullable Zig pointers
• normalize successful zero-length results to safe empty slices without changing public return types
• skip TypedArray alignment casts when the element length is zero
• cover shared null-plus-zero normalization in Zig and exercise empty and detached values through the example API