NSW pad trigger decoder: use return instead of throw in 2 places

These throw conditions, so far, have never been observed in data, due to how the data is handled upstream.

However, in case the conditions are satisfied in the future, it will be nicer to return an empty NSWPadTriggerL1a object instead of throw-ing. Additionally, the empty-ness of the object will be clear to users, so this won't cause problems downstream.

cc @sfrances