fix: apply OS detection when called without an options object

[yhuikzdtguioaert]

Fixes #133.

Problem

The default entry point auto-detects the OS to choose a default for the windows option, but that detection is skipped when picomatch() is called without an options object. The result then depends on whether an (empty) options object is passed:

// on Windows
picomatch('foo/*.js')('foo\\bar.js');     // false  (detection skipped)
picomatch('foo/*.js', {})('foo\\bar.js'); // true   (detection applied)

Both calls should behave the same.

Cause

In index.js the guard requires options to be truthy before defaulting windows:

if (options && (options.windows === null || options.windows === undefined)) {
  options = { ...options, windows: utils.isWindows() };
}

When options is undefined the whole condition is false, so windows is never defaulted and the matcher falls back to posix behavior even on Windows.

Fix

Also apply the default when no options object is given. { ...undefined } is an empty object, so the existing spread keeps working and the caller's object is never mutated. An explicit windows: true / windows: false is still respected.

Tests

Added two cases under options.windows in test/options.js. They stub utils.isWindows so the behavior is asserted deterministically on any platform (the first case fails without this change). Full suite is green (1979 passing).