Fail on blanks in passed env vars

Avoids to oversee pass_env/passenv going from blank to comma separated list
Resolves #2658

.github/PULL_REQUEST_TEMPLATE.md

@@ -3,7 +3,7 @@
 Please, make sure you address all the checklists (for details on how see
 [development documentation](http://tox.readthedocs.org/en/latest/development.html#development))!
 
-- [ ] ran the linter to address style issues (`tox -e fix_lint`)
+- [ ] ran the linter to address style issues (`tox -e fix`)
 - [ ] wrote descriptive pull request text
 - [ ] ensured there are test(s) validating the fix
 - [ ] added news fragment in `docs/changelog` folder

src/tox/tox_env/api.py

@@ -21,7 +21,7 @@
 from tox.execute.request import ExecuteRequest
 from tox.journal import EnvJournal
 from tox.report import OutErr, ToxHandler
-from tox.tox_env.errors import Recreate, Skip
+from tox.tox_env.errors import Fail, Recreate, Skip
 from tox.tox_env.info import Info
 from tox.tox_env.installer import Installer
 from tox.util.path import ensure_empty_dir
@@ -131,6 +131,12 @@ def register_config(self) -> None:
         )
 
         def pass_env_post_process(values: list[str]) -> list[str]:
+            blank_values = [v for v in values if " " in v or "\t" in v]
+            if blank_values:
+                raise Fail(
+                    f"pass_env/passenv variable can't have values containing "
+                    f"blanks like {blank_values}; a comma is possibly missing",
+                )
             values.extend(self._default_pass_env())
             return sorted({k: None for k in values}.keys())