Problem/Motivation
#3052147: comment_update_8701 fails if there are comments without field_name prevents sites from updating with problematic comments. @webchick had a great idea posted at the same time as that was committed.
This might be a silly question, but why can we not do a query like the one in #34 in the update if we detect this condition? Set field_name to "unknown" and programmatically create a stub entity with a machine name of "unknown" or something and assign entity_type there, flag an error in the logs and tell people to look at their comments and manually sort them out?
Proposed resolution
Try updating null values to placeholders that would be impossible via the UI or code - maybe something like %unknown%