python.el: Fix detection of native completion in Python 3 (bug #24401)

python.el: Fix detection of native completion in Python 3 (bug #24401)

With Python 3.5, (python-shell-completion-native-get-completions ... "")
would return an empty list, causing python.el to think that native
completion was unavailable (the difference between Python 2 and Python 3
is due to https://bugs.python.org/issue25660).

* lisp/progmodes/python.el (python-shell-completion-native-try): Use "_"
  to check whether native completion is available instead of "".