[displaymanager] Fix config schema

src/modules/displaymanager/displaymanager.schema.yaml

@@ -4,13 +4,17 @@ $id: https://calamares.io/schemas/displaymanager
 additionalProperties: false
 type: object
 properties:
-    "displaymanagers":
-        type: seq
-        sequence:
-          - { type: string, required: true, enum: [slim, sddm, lightdm, gdm, mdm, lxdm, kdm] }
-    "defaultDesktopEnvironment":
-        type: map
-        mapping:
-            "executable": { type: str }
-            "desktopFile": { type: str }
-    "basicSetup": { type: boolean, default: false }
+    displaymanagers:
+        type: array
+        items:
+            type: string
+            enum: [slim, sddm, lightdm, gdm, mdm, lxdm, kdm]
+        minItems: 1  # Must be non-empty, if present at all
+    defaultDesktopEnvironment:
+        type: object
+        properties:
+            executable: { type: string }
+            desktopFile: { type: string }
+        required: [ executable, desktopFile ]
+    basicSetup: { type: boolean, default: false }
+    sysconfigSetup: { type: boolean, default: false }