nixos/acme: Fix broken post-stop script

Previously the script would contain an empty `if` block (which is invalid
syntax) if both `data.activationDelay == null` and `data.postRun == ""`. Fix
this by adding a no-op `true`.

nixos/modules/security/acme.nix

@@ -266,6 +266,10 @@ in
                         systemctl --wait start acme-setlive-${cert}.service
                       fi
                       '' else data.postRun}
+
+                      # noop ensuring that the "if" block is non-empty even if
+                      # activationDelay == null and postRun == ""
+                      true
                     fi
                   '';