Handle nil return for delete label

[qian-squareup]

What and Why

Handle scenario where deleteLabelHandler return a nil Etre.Entity. This ensures that we're returning an error instead of a panic.

Testing

Confirmed that delete a required label now returns an error instead of panic.

Failed to delete label from elasticache_node 6884660f3c6561589dc92b5d: failed to update entity (db err: Document failed validation) (db-update-failed)