qt: fix basic usage when just `qt.enable = true` is set

modules/misc/qt.nix

@@ -159,8 +159,10 @@ in {
 
     # Necessary because home.sessionVariables doesn't support mkIf
     envVars = lib.filterAttrs (n: v: v != null) {
-      QT_QPA_PLATFORMTHEME =
-        styleNames.${cfg.platformTheme} or cfg.platformTheme;
+      QT_QPA_PLATFORMTHEME = if (cfg.platformTheme != null) then
+        styleNames.${cfg.platformTheme} or cfg.platformTheme
+      else
+        null;
       QT_STYLE_OVERRIDE = cfg.style.name;
     };
 
@@ -206,9 +208,10 @@ in {
     # Apply theming also to apps started by systemd.
     systemd.user.sessionVariables = envVars // envVarsExtra;
 
-    home.packages = (platformPackages.${cfg.platformTheme} or [ ])
-      ++ lib.optionals (cfg.style.package != null)
-      (lib.toList cfg.style.package);
+    home.packages = (lib.optionals (cfg.platformTheme != null)
+      platformPackages.${cfg.platformTheme})
+      ++ (lib.optionals (cfg.style.package != null)
+        (lib.toList cfg.style.package));
 
     xsession.importedVariables = [ "QT_PLUGIN_PATH" "QML2_IMPORT_PATH" ]
       ++ lib.optionals (cfg.platformTheme != null) [ "QT_QPA_PLATFORMTHEME" ]

tests/modules/misc/qt/default.nix

@@ -1,4 +1,5 @@
 {
+  qt-basic = ./qt-basic.nix;
   qt-platform-theme-gtk = ./qt-platform-theme-gtk.nix;
   qt-platform-theme-gnome = ./qt-platform-theme-gnome.nix;
 }

tests/modules/misc/qt/qt-basic.nix

@@ -0,0 +1,14 @@
+{ config, lib, pkgs, ... }:
+
+{
+  config = {
+    qt.enable = true;
+
+    nmt.script = ''
+      assertFileRegex home-path/etc/profile.d/hm-session-vars.sh \
+        'QT_PLUGIN_PATH'
+      assertFileRegex home-path/etc/profile.d/hm-session-vars.sh \
+        'QML2_IMPORT_PATH'
+    '';
+  };
+}