We had an online SharePoint list with a lookup that allowed for multiple selections. This was switched to single selection. In the service, our Power BI dataflow returned all list items with a selected lookup value as a single ID number, but any items that did not have any selected value were returned as an empty list rather than null. The Desktop, however, was returning null for these empty selections.
On the chance that this variance was a 1.0 vs 2.0 variance or that we would later decide to move object logic into a dataflow, we added logic on both platforms to convert the empty list items to null. For example, if the column with mixed types was named [OldMultiSelect], the lazy way to avoid needing to change downstream code is
1. added column with logic
if Value.Is( [OldMultiSelect], type list) then null else [OldMultiSelect]
2. removed [OldMultiSelect] column
3. renamed new column from #1 above to [OldMultiSelect]
