For instance marc_tag_structure.authorised_value can be "" or NULL (not coming from bug 24594).
Created attachment 116192 [details] [review] Bug 27591: Set biblio framework AV to NULL instead of empty string
There is more to investigate here. We should also remove the entry from the yml and let the DBMS set the default value.
Appears to still be valid: installer/data/mysql/en/marcflavour/marc21/mandatory/authorities_normal_marc21.yml: authorised_value: installer/data/mysql/en/marcflavour/marc21/mandatory/authorities_normal_marc21.yml: authorised_value: installer/data/mysql/en/marcflavour/marc21/mandatory/authorities_normal_marc21.yml: authorised_value: installer/data/mysql/en/marcflavour/marc21/mandatory/authorities_normal_marc21.yml: authorised_value: ...