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
```

Lib/venv/scripts/common/activate

@@ -28,7 +28,7 @@ deactivate () {
     fi
 
     unset VIRTUAL_ENV
-    if [ ! "$1" = "nondestructive" ] ; then
+    if [ ! "${1:-}" = "nondestructive" ] ; then
     # Self destruct!
         unset -f deactivate
     fi