Make zero-shard SELECT consistent with CitusDB

Making a query against a table that has been marked as "distributed" (either by `CREATE TABLE ... DISTRIBUTE BY ...` or using `master_create_distributed_table`) which does not yet have any shards (created using either `\stage` or `master_create_worker_shards`) results in different behavior depending upon whether the active planner is CitusDB's or `pg_shard`'s:
- Under CitusDB, the query returns zero rows
- `pg_shard` raises an error directing the user to call `master_create_worker_shards`

The latter is obviously helpful under `pg_shard` (especially for users who have missed this step), but is confusing under CitusDB. Now that integration is tighter, we need to figure out what the best approach is.