bpo-37885: venv: Don't produce unbound variable warning on deactivate (GH-15330)

bpo-37885: venv: Don't produce unbound variable warning on deactivate (GH-15330)

Before, running deactivate from a bash shell configured to treat undefined variables as errors (`set -u`) would produce a warning:

```
$ python3 -m venv test
$ source test/bin/activate
(test) $ deactivate
-bash: $1: unbound variable
```