Support shells with "set -u"

[TomiBelan]

Currently the nodeenv activation code cannot run in bash/zsh shells with "set -u" enabled. This PR fixes it.

Usually you activate virtual environments in an interactive shell, but sometimes it is useful to do it inside a shell script. When writing shell scripts it is often recommended to enable additional strict error checking options, for example set -e -u -o pipefail. -u means reading undefined variables is an error.

It is based on the equivalent change in CPython:

• PR: bpo-25351: avoid activate failure on strict shells python/cpython#3804
• Bug: pyvenv activate script failure with specific bash option python/cpython#69538
• PR2: bpo-37885: venv: Don't produce unbound variable warning on deactivate python/cpython#15330
• Bug2: venv: Don't produce unbound variable warning on deactivate python/cpython#82066

Example before this PR:

$ python3 -m venv test1
$ test1/bin/pip install nodeenv
Collecting nodeenv
  Using cached nodeenv-1.8.0-py2.py3-none-any.whl (22 kB)
Requirement already satisfied: setuptools in ./test1/lib/python3.10/site-packages (from nodeenv) (59.6.0)
Installing collected packages: nodeenv
Successfully installed nodeenv-1.8.0
$ test1/bin/nodeenv -p
 * Install prebuilt node (21.2.0) ..... done.
 * Appending data to /home/tomi/tmpt/nodeenv/test1/bin/activate
 * Appending data to /home/tomi/tmpt/nodeenv/test1/bin/activate.fish
$ test1/bin/npm
ERROR: npm v10.2.3 is known not to run on Node.js v12.22.9.  This version of npm supports the following node versions: `^18.17.0 || >=20.5.0`. You can find the latest version at https://nodejs.org/.
...
$ bash -c 'source test1/bin/activate; npm --version'
10.2.3
$ bash -c 'set -u; source test1/bin/activate; npm --version'
test1/bin/activate: line 82: _OLD_NODE_VIRTUAL_PATH: unbound variable

After this PR it works properly:

$ test3/bin/pip install ./nodeenv/
...
$ test3/bin/nodeenv -p
...
$ bash -c 'source test3/bin/activate; npm --version'
10.2.3
$ bash -c 'set -u; source test3/bin/activate; npm --version'
10.2.3