`LIKE ... ESCAPE <escape_char>` should accept an empty string

Currently, we use `Option` to represent `escape_char`, which cannot represent three case. We should introduce a new enum to represent that.

```rust
enum EscapeChar {
NotSpecified,
Empty,
Escape(char),
}
```

to pass the case:

https://www.postgresql.org/docs/current/functions-matching.html#FUNCTIONS-LIKE:~:text=it's%20also%20possible%20to%20select%20no%20escape%20character%20by%20writing%20escape%20''

未指定任何文件、测试或入口点。首先从 `Option` 开始，跟踪 `escape_char` 的 Rust 表示经过解析的过程，然后检查 PostgreSQL 的 `ESCAPE ''` 是如何表示和测试的；当空、未指定和字符这几种情况仍保持区分时，即表示完成。

由索引模型根据 Issue 内容生成。