Default values are ignored in AppendStruct

### Issue description
Altering table to add a new column causes an error

```
[AppendStruct]: missing destination name "" in "
```

even if I've specified a default value for the column.

Perhaps checking that column has a default value here might solve the issue.

https://github.com/ClickHouse/clickhouse-go/blob/ee79713daf7a10ba3b363797f74b0909be463b49/struct_map.go#L69-L74

### Configuration
*OS:* unrelated

*Interface:* E.g. `native`

*Driver version:* v2.0.12

*Go version:* unrelated

*ClickHouse Server version:* 20.10.6.27