This question is now almost 6 months old. Is there seriously no one out there can can answer a question as simple as this? Is McAfee's ESM incapable of filtering by null/not null values?
If either of these are the case, I will be making a strong case to abandon ESM/Nitro, and opt for a SIEM with either better support or a more-knowledgeable community.
I haven't looked at the Destination or Source IP Address fields, but I have found something interesting for the Source User field.
Click the ! (for NOT) for the Source User field, then type "regex($)" in the field -- this will show you all events with a Null Source User. Unclick NOT to see events with a Source User that is not null/blank/empty.
I've only tested this on Windows events currently. Stumbled across that after all the standard null expressions failed to work (\x00, \x0, \000, ^$, \A\z, and many others).
Regarding Rickgrimes' post, the Nitro/McAfee SIEM really doesn't play well with "null" unfortunately (pretends the field simply doesn't exist -- can't filter on something that doesn't exist). There might be a PCRE setting McAfee could adjust... if we put in Product Enhancement Requests, assuming there isn't a technological reason why it is the way it is.
Using contains([^a-zA-Z0-9\$\-\_]*) may also work, as it is looking for anything matching special characters only, not including the hyphen, underscore, and the dollar (windows system accounts), which should be the only time the username string actually contains a special character, unless the parser grabs a username with a % token in it, which I have seen a couple times.
contains([a-zA-Z0-9\$\-\_]*) and contains([^a-zA-Z0-9\$\-\_]*) do not work, the ESM considers them invalid expressions. However, Source User NOT regex([a-zA-Z0-9\$\-\_]*) does work and seems to return the similar or same results as Source User NOT regex($) -- though like you said not all special characters are excluded so it's possible a Source User field consisting of just those characters (e.g. %#@) might be returned.
So far these filters work on Windows and Linux data sources that I've seen and tested.