[fix] 条件分支为空时, 构建sql错误

engine/engine_case.go

@@ -11,6 +11,10 @@ type EngineCase[T any] struct {
 }
 
 func (this EngineCase[T]) ToSql(param contracts.ModelParam) db.Expression {
+	if this.predicate == nil || this.predicate.IsEmpty() {
+		return db.Raw("NULL")
+	}
+
 	return this.predicate.ToSql(param)
 }