[3.13] gh-146310: Fix ensurepip to treat empty WHEEL_PKG_DIR as unset (GH-146357) (#146535)

gh-146310: Fix ensurepip to treat empty WHEEL_PKG_DIR as unset (GH-146357)

Path('') resolves to CWD, so an empty WHEEL_PKG_DIR string caused
ensurepip to search the current working directory for wheel files.
Add a truthiness check to treat empty strings the same as None.
(cherry picked from commit 73cc1fd4f45b4daf2b2f9a6be69148775c7c2bff)

Co-authored-by: Imgyu Kim <kimimgo@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>