master_create_empty_shard should take regclass

`master_create_empty_shard` currently uses `text` as the first argument, it should use `regclass` instead.

Start by locating the master_create_empty_shard function definition and checking how its first argument is declared and used. Update the argument type to regclass, then verify that the function accepts a relation name through PostgreSQL's regclass handling and that existing behavior remains intact.

Written by the indexing model from the issue text.