v0.2.1 — fix fabricated types path for typeless export subpaths

Fixed

• Broken types path for typeless export subpaths — An exports entry with no types condition and no matching entry (e.g. a UMD-only subpath like "./umd": {"require": "./dist/umd.js"}) came out of the build as {"types": "./undefined.d.ts", ...}, pointing at a nonexistent file. Fabricated types conditions are now only emitted when the corresponding .d.ts actually exists, and exports with no entry to fill from pass through untouched.