feat: Introduce Undefined function for Nullable type and refactor DTOs to use Nullable directly

backend/internal/types/nullable.go

@@ -18,6 +18,10 @@ func NewNullable[T any](value T) Nullable[T] {
 
 // Null erstellt eine leere Nullable-Instanz (ungesetzt)
 func Null[T any]() Nullable[T] {
+	return Nullable[T]{Valid: true}
+}
+
+func Undefined[T any]() Nullable[T] {
 	return Nullable[T]{Valid: false}
 }