`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''

未指定任何檔案、測試或進入點。首先從 `Option` 開始，追蹤 `escape_char` 的 Rust 表示經過剖析的過程，然後檢查 PostgreSQL 的 `ESCAPE ''` 如何表示及測試；當空白、未指定和字元這幾種情況仍保持區分時，即表示完成。

由索引模型根據 Issue 內容生成。