run-tests: avoid set PYTHONUSERBASE environment variable to None
According to python documentation, get_config_var can return None and it was
during running mercurial tests under a virtual environment with the local built
mercurial installed as a python package with `pip install -e .`.
On the other hand, if mercurial is installed in the user library folders with
`pip install --local -e .` the issue doesn't reproduce.
The fallback is that it will just set an empty string and everything should work
as before, according to https://www.python.org/dev/peps/pep-0370/:
The path to the user base directory can be overwritten with the environment
variable PYTHONUSERBASE. The default location is used when PYTHONUSERBASE is
not set or empty.