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.

tests/run-tests.py

@@ -1021,7 +1021,7 b' class Test(unittest.TestCase):'
             offset = '' if i == 0 else '%s' % i
             env["HGPORT%s" % offset] = '%s' % (self._startport + i)
         env = os.environ.copy()
-        env['PYTHONUSERBASE'] = sysconfig.get_config_var('userbase')
+        env['PYTHONUSERBASE'] = sysconfig.get_config_var('userbase') or ''
         env['HGEMITWARNINGS'] = '1'
         env['TESTTMP'] = self._testtmp
         env['HOME'] = self._testtmp