Clickhouse: password is optional but we try to access it anyway

For Clickhouse type data sources, we don't require a password. But the code does require it by trying to directly access the value in the options dictionary, instead of using `get`:

https://github.com/getredash/redash/blob/823e4ccdd6fcfee5d0df0d919d87af3100876549/redash/query_runner/clickhouse.py#L77