[pg] Support doubled public in SET search_path

Run local YDB https://github.com/ydb-platform/ydb/wiki/Local-run-postgres-tests

```
echo "SET search_path = \"public\", public" | psql postgres://root:1234@localhost:5432/local
```

Result:
```
Status: INTERNAL_ERROR
Issues:
<main>: Error: Error while parsing query.
    <main>:1:1: Error: VariableSetStmt, expected 1 arg, but got: 2
```

Expected result:
```
SET
```