REL-1988: OT Java NPE when moving Offset iterator

This is simple PR to check for nulls an a NIRI Phase 2 check

On other instruments the same logic is followed, if the exposure time is null, ignore.
On the other hand there is a different possible strategy. If observe:exposureTime is null, then we try with instrument:exposureTime, if that is still null we ignore.
Do you think this second strategy would be better?