Suite shard plan requests without enumerated suites return a plan instead of a 500

Requests for a suite-granularity shard plan no longer fail with a 500 when the client doesn’t enumerate suites. If the request arrives with test_suites unset, Tuist now derives the suite list from server-side test history and returns a valid shard plan instead of crashing on a nil unit list. The module-granularity path receives the same nil-safety treatment so it can’t hit the same crash.