Such records are commonly used in data warehouses, because NULL values are usually ambiguous. Creating dummy records for various unusual circumstances is preferred, to eliminate this ambiguity. In your case, some kind of "guest" credential would be perfectly appropriate.
That said, if you expect this to be the only reason for an action to not have a credential, using NULL values is fine, too. It's reasonably intuitive: this action doesn't have any creds, but did occur, I guess the user didn't need to authenticate.
You'll need to weigh the (modest) cost of implementing a guest credential versus the chance of confusion from using NULL values.