Actually filter out missing interpreters

This fixes pypa#5261.

Before this change, I would get a "The Python interpreter can't be
found" error when running `pipenv install --system` with a python3 but
no python.

Demo:

```
$ docker run $(docker build -q -f Dockerfile.pipenv-2022.8.15 https://github.com/jfly/2022-08-16-pipenv-system-which-issue.git#main)
Installing dependencies from Pipfile.lock (63bec0)...
The Python interpreter can't be found.▉ 0/4 — 00:00:00
  🐍   ▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉ 0/4 — 00:00:00
```

I'm confident this bug was introduced in
pypa@f276360,
which removed a usage of `first()`, but broke the "filter None values"
behavior that `first()` gave us.

pipenv/utils/shell.py

@@ -446,6 +446,7 @@ def project_python(project, system=False):
         python = project._which("python")
     else:
         interpreters = [system_which(p) for p in ("python", "python3")]
+        interpreters = [i for i in interpreters if i]  # filter out not found interpreters
         python = interpreters[0] if interpreters else None
     if not python:
         click.secho("The Python interpreter can't be found.", fg="red", err=True)