This would be useful for testing #6773. This is because, given that
#6773
only adds support for router / fast-path queries, theoretically almost
all
the tests that we have in that test file should work for null-shard-key
tables too (and they indeed do).
I deliberately did not replace multi_router_planner_fast_path.sql with
the one that I'm adding into arbitrary configs because we might still
want to see when we're able to go through fast-path planning for the
usual distributed tables (the ones that have a shard key).
Onur Tirtir