Feature request or enhancement

For feature requests or enhancements, provide this information, please:

Which use case/requirement will be addressed by the proposed feature?

It is possible for a keyspace to have some shards that have no tablets. In this case, Debezium should gracefully handle this by doing two things:

1. Routing queries to shards with live tablets, otherwise it gets this error

java.lang.AssertionError: Error response: error {  message: "target: <keyspace>.1184-1185.primary: no healthy tablet available for \'keyspace:\"keyspace\" shard:\"1184-1185\" tablet_type:PRIMARY\'"  code: UNAVAILABLE}

     2. If a shard list is not specified, it should detect and include all non-empty shards (shards with tablets).

Implementation ideas (optional)

Will put out a PR