I have confirmed through GS Support that there is no way currently to set a filter to test for nul values in an Advanced Outreach.



While you can do so in Power Lists, there are situations when you would want to do such validation in the outreach instead. For instance, to pick a different variant if a token is empty rather than create a series of mailings. Also, if you are using the value of this token as part of an email as you cannot default a space in a token. With AO stronger testing capabilities in email chains, this can also be very helpful to take a different path. 



If you share the same interest or you see the potential for your use cases, show interest to this post to help  prioritize it. Thanks!