Make zero-shard SELECT consistent with CitusDB

[jasonmp85]

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.