The creator column in the CONTENT_PERM should not be null unless the user that caused that record to be added was really anonymous.

For reference see:
https://support.atlassian.com/browse/CSP-95615
https://answers.atlassian.com/questions/133543/space-admin-page-restrictions-creator-shows-as-anonymous-when-it-isn-t-bug

To confirm, Adrien Ragot (Atlassian) in answers confirmed it was a bug:

"I have just noticed a small bug: When you use the Restrictions dialog to add several groups, the first group always appears in CONTENT_PERM with CREATOR = NULL and all the other groups have a value in CREATOR."

Bruna Griebeler (Atlassian) also mentioned was a bug when she said in the support ticket:

"As we talked yesterday, this is the query to set the creator as the lastmodifier user, to avoid let the creator as null, for security reasons.

update CONTENT_PERM set CREATOR=LASTMODIFIER where creator=NULL;

PS: Please create a backup of your database before running that query!!
About the bug itself, I'll talk with a developer and will let you know as soon as possible."

In the support ticket, the customer indicated that they would like to see a resolution that does not involve having to run a SQL update frequently.

Without a customer workaround, this bug basically disables the ability for the space administrator to audit changes properly.