Describe the solution you'd like

Null search attribute values can be produced in a few ways, namely because upsert search attributes puts them in workflow info (which arguably it shouldn't, but that's a separate discussion) and we reuse that map for continue as new. We should stop sending null search attributes on start or continue as new. Don't forget to mark this as 💥 backwards incompatible since it does very slightly change what attributes appear in a new workflow in this rare case.