(ITS#8266) empty ldapmodify refused with slapo-unique

[geert@hendrickx.be]

Full_Name: Geert Hendrickx
Version: 2.4.42
OS: 
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (212.123.14.2)


When slapo-unique constraints are in effect, it seems empty updates are
no longer allowed:

$ ldapmodify -x -h localhost -D cn=Manager,dc=my-domain,dc=com -w secret
dn: cn=test1,dc=my-domain,dc=com
changetype: modifyA%A
modifying entry "cn=test1,dc=my-domain,dc=com"
ldap_modify: Invalid syntax (21)
        additional info: unique_modify() got null op.orm_modlist

Why is this considered invalid syntax?  Without slapo-unique constraint,
empty updates like these are accepted.