gh-146310: Fix ensurepip to treat empty WHEEL_PKG_DIR as unset (#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.

Co-authored-by: Victor Stinner <vstinner@python.org>