fix(optimizer): skip null-valued exports in expandGlobIds glob resolution

[LeSingh1]

Fixes #22586.

When using glob patterns in optimizeDeps.include (e.g. my-pkg/*), Vite expands the pattern against the package's exports field. The non-glob path of that expansion unconditionally added every export key to the candidate list, including keys with a null value.

Per the Node.js docs, a null export value is how packages mark subpaths as intentionally private and unreachable:

To exclude private subfolders from patterns, null targets can be used.
https://nodejs.org/api/packages.html#subpath-patterns

For example, a package with this in its package.json:

{
  "exports": {
    ".": "./dist/index.js",
    "./utils": "./dist/utils.js",
    "./internal": null
  }
}

When a user adds "my-pkg/*" to optimizeDeps.include, the old code would include my-pkg/internal in the resolved list. Vite then tries to bundle that path, fails to find a file for it, and throws a build error.

The glob branch already handled this correctly by calling getFirstExportStringValue which returns undefined for null, triggering an early continue. The non-glob branch just needed the same null check.

The fix is a single guard in expandGlobIds inside packages/vite/src/node/optimizer/resolve.ts:

if (exports[key] === null) continue

A unit test is added that confirms null-valued export keys are excluded from the resolved list while non-null ones are still included.