Imap-Lo - no direct events cause crash in l1a processing

[vineetbansal]

This block in l1a.py crashes and causes no products being created if de_count is 0.

    if LoAPID.ILO_SCI_DE in datasets_by_apid:
        logger.info(
            f"\nProcessing {LoAPID(LoAPID.ILO_SCI_DE).name} "
            f"packet (APID: {LoAPID.ILO_SCI_DE.value})"
        )
        logical_source = "imap_lo_l1a_de"
        ds = datasets_by_apid[LoAPID.ILO_SCI_DE]

        # For segmented packets, combine raw bytes directly
        # Always call combine_segmented_packets to set MET field
        ds = combine_segmented_packets(ds)

        ds = parse_events(ds, attr_mgr)
        ds = add_dataset_attrs(ds, attr_mgr, logical_source)
        datasets_to_return.append(ds)