Empty table overrides are formatted without any parenthesises,
but they are required by a parser,
and it is not possible to parse empty table overrides without it.
:) CREATE DATABASE db ... TABLE OVERRIDE t1()
CREATE DATABASE db
...
TABLE OVERRIDE `t1`
This query will be saved to metadata and ClickHouse will not be able to start up, since
table overrides require ().
Example:
```sql
EXPLAIN TABLE OVERRIDE mysql('127.0.0.1:3306', 'db', 'table', 'user', 'pw')
PARTITION BY tuple(toYYYYMM(created), id % 8)
```
Validations done:
* check that storage overrides do not reference unknown or nullable columns
* check that default specifier is not modified for columns
