Skip lock acquisition in CrewTrainingHandler.load when file is missing (#5935)

Every agent kickoff calls _use_trained_data, which calls
CrewTrainingHandler(...).load(). Since #4827 wrapped load() in store_lock,
that means every kickoff acquires the cross-process (Redis-backed when
REDIS_URL is set) lock even on deployments that never train and have no
trained-agents file on disk.

Move the missing/empty-file short-circuit above store_lock so the lock is
only acquired when there is actually a file to read. save() and the real
read remain locked.