Problem/Motivation
If you want to skip on empty values, such as a NULL value - like SkipOnEmpty documents -, then you get an error "Did not save to map table due to NULL value for key field" instead of an ignored row/process.
So you can not actually skip a row/process on a NULL value.
(In my case, I'm using the spreadsheet source plugin from the migrate_spreadsheet module and my spreadsheet has an empty cell in a column that holds a required value, so I want to skip it when empty.)
Proposed resolution
Don't consider a NULL value as an indication of a problem, by default, in contradiction to what \Drupal\migrate\Plugin\migrate\id_map\Sql\saveIdMapping() currently mentions.
Check whether a process plugin allows a NULL value or - in the case of skip_on_empty - a process plugin wants to skip - not throw an error - on a NULL value.
Remaining tasks
Patch.
User interface changes
None.
API changes
None that I know of.
Data model changes
Does the sourceid1 column in migrate_map_* tables need to allow NULL values?
