fix: primitive() and enum validation now correctly handle null

[SrHenry]

Summary

Fixes #31

The primitive() schema guard and validateDefault both used Generics.Primitives.includes(typeof arg) to check if a value is a primitive. Since typeof null === "object" (not "null"), null was never matched despite being part of the PrimitiveType union.

Changes

Root cause fix — src/validators/schema/primitive.ts

Added explicit arg === null check before the typeof check in the guard:

branchIfOptional(arg, []) || arg === null || (Generics.Primitives as readonly string[]).includes(typeof arg)

This mirrors the pattern used in asNull.ts which already checks arg === null directly.

Same structural bug in validator — src/validators/schema/helpers/validate/validateDefault.ts

The validateDefaultInner function had the same typeof-only check in two places:

1. primitive case (line 68) — Added ctx.arg !== null && guard before the typeof check
2. enum case (line 189) — Same fix applied. This was an additional bug with the same root cause: asEnum schemas containing null values would fail validation because the enum case in validateDefault rejects non-typeof-matchable values before checking enum members.

Note: The asEnum.ts guard itself delegates to primitive(), so it is automatically fixed by the primitive.ts change. The separate fix was only needed in validateDefault.ts.

Tests — src/validators/__tests__/primitive.spec.ts

Replaced the skipped stub with a full test suite covering:

• All primitive types including null (the core bug)
• Non-primitive rejection
• optional() variant
• validator() method existence

Reproduction (before fix)

primitive()(null)       // false — now true
primitive().optional()(null)  // false — now true