Regression in parsing `NULL LIKE <expr>`

[andygrove]

This test passes against 0.20.0 but fails in the main branch. This is likely caused by my recent changes for parsing LIKE.

#[test]
fn parse_null_like() {
    let sql =
        "SELECT \
            column1 LIKE NULL AS col_null, \
            NULL LIKE column1 AS null_col \
        FROM customers";
    let _ = verified_only_select(sql);
}

Failure:

SQL(ParserError(\"Expected a concrete value, found: column1\"))