Update nox/manifest.py

Co-authored-by: Claudio Jolowicz <cjolowicz@gmail.com>

nox/manifest.py

@@ -190,12 +190,11 @@ def make_session(
             extra_pythons = self._config.extra_pythons  # type: List[str]
             if isinstance(func.python, (list, tuple, set)):
                 func.python = list({*func.python, *extra_pythons})
-            elif not multi and func.python:
+            elif not multi and func.python is not False:
                 # if this is multi, but there is only a single interpreter, it
                 # is the reentrant case. The extra_python interpreter shouldn't
                 # be added in that case. If func.python is False, the session
-                # has no backend; treat None and the empty string equivalently.
-                # Otherwise, add the extra specified python.
+                # has no backend. Otherwise, add the extra specified python.
                 assert isinstance(func.python, str)
                 func.python = list({func.python, *extra_pythons})