`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''

No files, tests, or entry points are named. Start by tracing the Rust representation of `escape_char` from `Option` through parsing, then inspect how PostgreSQL `ESCAPE ''` is represented and tested; done means the empty, unspecified, and character cases remain distinct.

Written by the indexing model from the issue text.