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

[kimimgo]

Fixes #146310

Path('') resolves to the current working directory (PosixPath('.')), so when WHEEL_PKG_DIR is an empty string (not None), ensurepip searches CWD for wheel files. This can cause unexpected behavior or crashes if CWD contains files matching pip-*.whl.

Changes

• Lib/ensurepip/__init__.py: Add truthiness check so empty strings are treated the same as None
• Lib/test/test_ensurepip.py: Add test verifying empty WHEEL_PKG_DIR does not search CWD

Before/After

# Before: Path('').resolve() -> PosixPath('/current/working/dir')
# Empty string was treated as a valid directory path

# After: Empty string treated as None -> use bundled wheel

• Issue: ensurepip looks for wheels in CWD when WHEEL_PKG_DIR is an empty string #146310