nixos/systemd-lib: fix building of empty unit files

This is a fixup for c1ae82f448.

nix' `passAsFile` does not create empty files for variables that are
`null`.

This results in the following error for units that have no overrides or
content, but are, e.g. `wantedBy`:
`mv: cannot stat '': No such file or directory`.

Minimal reproducer:
`systemd.units.empty.wantedBy = [ "multi-user.target" ];`

This is often necessary when a unit is loaded in via `systemd.packages`.

nixos/lib/systemd-lib.nix

@@ -20,7 +20,10 @@ in rec {
       pkgs.runCommand "unit-${mkPathSafeName name}"
         { preferLocalBuild = true;
           allowSubstitutes = false;
-          inherit (unit) text;
+          # unit.text can be null. But variables that are null listed in
+          # passAsFile are ignored by nix, resulting in no file being created,
+          # making the mv operation fail.
+          text = optionalString (unit.text != null) unit.text;
           passAsFile = [ "text" ];
         }
         ''