nixos/stash: fix empty immutable plugins (#402574)

nixos/modules/services/web-apps/stash.nix

@@ -324,7 +324,6 @@ let
       '';
       apply =
         srcs:
-        optionalString (srcs != [ ]) (
         pkgs.runCommand "stash-${kind}"
           {
             inherit srcs;
@@ -332,6 +331,8 @@ let
             preferLocalBuild = true;
           }
           ''
+            mkdir -p $out
+            touch $out/.keep
             find $srcs -mindepth 1 -name '*.yml' | while read plugin_file; do
               grep -q "^#pkgignore" "$plugin_file" && continue
 
@@ -363,8 +364,7 @@ let
                   .files = (strenv(plugin_files) | split("\n"))
                 ' > $out_path/manifest
             done
-            ''
-        );
+          '';
     };
 in
 {