impr(handle-null-job): ignore not found jobs

[mhshiba]

Fix this bug:

TypeError
TypeError: Cannot read properties of undefined (reading 'name')
    at QueueEvents.<anonymous> (/src/bull/bullmq-metrics.factory.ts:252:37)
    at processTicksAndRejections (node:internal/process/task_queues:95:5)

IDK why a completed job can't be found, but it makes sense to me to add this safe guard.