A CREATE KEYSPACE query which specifies an empty string ('') as the replication factor value is currently allowed:
```cql
CREATE KEYSPACE bad_ks WITH replication = {'class': 'NetworkTopologyStrategy', 'replication_factor': ''};
```
This is wrong, it's invalid to have an empty replication factor string.
It creates a keyspace without any replication, so the tables inside of it aren't writable.
Trying to create a `SimpleStrategy` keyspace with such replication factor throws an error, `NetworkTopolgyStrategy` should do the same.
The problem was in `prepare_options`, it treated an empty replication factor string as no replication factor.
Changing it to `std::optional` fixes the problem,
Now `std::nullopt` means no replication factor, and `make_optional("")` means that there is a replication factor, but it's described by an empty string.
Fixes: https://github.com/scylladb/scylladb/issues/13986Closes#13988
* github.com:scylladb/scylladb:
test/network_topology_strategy_test: Test NTS with replication_factor option in test_invalid_dcs
ks_prop_defs: disallow empty replication factor string in NTS
7cdee303cf