Eval does not stop on the first assertion failure it encouters.
Instead, it tries to evaluate all assertions and returns with a list of
those that failed.
This means our very top `config.mailserver.stateVersion != null`
assertion does not gate against any other assertions trying to compare
null against an integer.
The error prior to this commit can be reproduced by removing
`mailserver.stateVersion = 999;` in tests/lib/config.nix and then trying
to evaluate any of the tests:
~~~bash
# nix eval --raw .#checks.x86_64-linux.internal-unstable
error:
… while evaluating the attribute 'outPath'
at /nix/store/syvnmj3hhckkbncm94kfkbl76qsdqqj3-source/lib/customisation.nix:421:7:
420| drv.drvPath;
421| outPath =
| ^
422| assert condition;
… while calling the 'getAttr' builtin
at «internal»:1:500:
(stack trace truncated; use '--show-trace' to show the full trace)
error: cannot compare null with an integer
~~~
6ef1eb9ce1