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

[TennyZhuang]

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

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''

[TennyZhuang]

I can help to implement that.

[TennyZhuang]

Another option is create a newtype EscapeChar to represent '' or a single character, and store an Option<EscapeChar> in the AST.

[TennyZhuang]

I'd like to port risingwavelabs/risingwave@b05cb06 back to the repo, if the fix is reasonable.

[alamb]

Seems resonable to me