Security access entries for guests aren't cached in the security cache

Since fixing XWIKI-18508, access right checks for public access to the wiki aren't cached anymore. This could lead to slightly degraded performance.

The reason for this is that the cache now checks that for security access entries, the indicated user parent entry is actually a user entry but at the same time, loading this user entry fails for the guest user. The reason for this is that the DefaultSecurityEntryReader doesn't load any rules for the guest user and then insertion into the security cache fails because a main wiki entry with rules already exists.