AppManager: Fix derived value calculation

Change-Id: I97c429437b5df4ea782883190f420f9f60111ff4
diff --git a/core/installer/app.go b/core/installer/app.go
index c4604c8..e054964 100644
--- a/core/installer/app.go
+++ b/core/installer/app.go
@@ -734,8 +734,11 @@
 	if err != nil {
 		return EnvAppRendered{}, err
 	}
+	if dv == nil {
+		dv = map[string]any{}
+	}
 	mv := merge(dv.(map[string]any), values)
-	derived, err := deriveValues(mv, values, a.Schema(), networks, clusters, vpnKeyGen)
+	derived, err := deriveValues(mv, mv, a.Schema(), networks, clusters, vpnKeyGen)
 	if err != nil {
 		return EnvAppRendered{}, err
 	}