Fix ValueError: max() arg is an empty sequence on (tox-dev#1529)

* fix ValueError: max() arg is an empty sequence

When running `tox -l` with a tox.ini file not containing any default environments, instead of an empty output, an exception is raised:

```
$ tox -l
tox -l
Traceback (most recent call last):
  File "/Users/jq/.pyenv/versions/py38/bin/tox", line 8, in <module>
    sys.exit(cmdline())
  File "/Users/jq/.pyenv/versions/3.8.1/envs/py38/lib/python3.8/site-packages/tox/session/__init__.py", line 44, in cmdline
    main(args)
  File "/Users/jq/.pyenv/versions/3.8.1/envs/py38/lib/python3.8/site-packages/tox/session/__init__.py", line 68, in main
    exit_code = session.runcommand()
  File "/Users/jq/.pyenv/versions/3.8.1/envs/py38/lib/python3.8/site-packages/tox/session/__init__.py", line 189, in runcommand
    self.showenvs(all_envs=False, description=show_description)
  File "/Users/jq/.pyenv/versions/3.8.1/envs/py38/lib/python3.8/site-packages/tox/session/__init__.py", line 293, in showenvs
    show_envs(self.config, all_envs=all_envs, description=description)
  File "/Users/jq/.pyenv/versions/3.8.1/envs/py38/lib/python3.8/site-packages/tox/session/commands/show_env.py", line 14, in show_envs
    max_length = max((len(env) for env in (default + extra)))
ValueError: max() arg is an empty sequence
```

* trying to follow contributing guidelines

* add test, fails without merge, succeeds after

Signed-off-by: Bernat Gabor <[email protected]>

docs/changelog/1343.bug.rst

@@ -0,0 +1 @@
+Fix ValueError on ``tox -l`` for a ``tox.ini`` file that does not contain an ``envlist`` definition. - by :user:`jquast`

src/tox/session/commands/show_env.py

@@ -11,7 +11,7 @@ def show_envs(config, all_envs=False, description=False):
 
     if description and default:
         report.line("default environments:")
-    max_length = max(len(env) for env in (default + extra))
+    max_length = max(len(env) for env in (default + extra) or [""])
 
     def report_env(e):
         if description:

tests/unit/session/test_list_env.py

@@ -216,3 +216,13 @@ def test_listenvs_all_extra_definition_order_increasing(cmd, initproj):
     result = cmd("-a")
     expected = ["py36", "a", "b"]
     assert result.outlines == expected
+
+
+def test_listenvs_without_default_envs(cmd, initproj):
+    """When running tox -l without any default envirinments, nothing happens."""
+    initproj(
+        "logsnada", filedefs={"tox.ini": ""},
+    )
+    result = cmd("-l")
+    assert result.ret == 0
+    assert result.out == ""