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

Creating a shard plan with suite granularity and no enumerated test suites no longer fails with a 500 “unknown server response” error. After a previous change that stopped the CLI from booting every test bundle to enumerate suites, these requests arrived with the test_suites field present but nil, which slipped past the old empty-list guard and crashed the server. The shard planning code now correctly pattern-matches a non-empty list for test_suites and applies the same nil-safe treatment to modules, so the server falls back to deriving suites from history and returns a valid shard plan (with shard_count and shards) instead of erroring out.