Specifying destdir as empty string leads to no scripts installed

[dvzrv]

Hi! Recently I have been experimenting a bit with build and installer in the context of CI pipelines.

I noticed that if python -m installer --destdir="" <wheel> at least scripts (but maybe also the rest of the wheel contents) are not installed to / but likely somewhere else.
From a user perspective I would assume that providing empty string would behave the same way as not providing --destdir at all (in which case files are properly installed to the root filesystem).

When looking at e.g. how this is handled in a Makefile which allows overriding of the destination using the DESTDIR environment variable we can observe that the following code snippet follows my above assumption:

PREFIX ?= /usr/local

install:
    install -vDm 644 some/file -t "$(DESTDIR)/$(PREFIX)/share/foo/"

[pradyunsg]

> /Users/pradyunsg/Developer/github/installer/src/installer/destinations.py(145)_path_with_destdir()
-> return os.path.join(self.destdir, rel_path)
(Pdb) locals()
{'self': <installer.destinations.SchemeDictionaryDestination object at 0x107612a90>, 'scheme': 'purelib', 'path': 'installer/__init__.py', 'file': '/Users/pradyunsg/Developer/github/installer/.venv/lib/python3.11/site-packages/installer/__init__.py', 'file_path': PosixPath('/Users/pradyunsg/Developer/github/installer/.venv/lib/python3.11/site-packages/installer/__init__.py'), 'rel_path': PosixPath('Users/pradyunsg/Developer/github/installer/.venv/lib/python3.11/site-packages/installer/__init__.py')}
(Pdb) self.destdir
''

Well, that's hilarious. You're getting os.path.join("", <path-without-anchor>). A PR improving the behaviour around this would be welcome! :)