PowerBi Preexisting Filters and Is Not Blank

Hello,

I have been using PowerBI premiium embeded in our app for the last about year now.  It has been working well mostly, but there are a few quirks and I just found another I am not sure how to solve.  I have been passing in filters just fine, both Advanced and Basic, and found today a report that was not filtering when I passed them in. 

The reports filters quit working when I had a preexisitng filter where it was taking out all the blank values for a field.  In my filters, I make sure to read out existing filters, and then pass them, along with the new ones, back in.  However, when it gets passed in it has the value of [null] set for values and this seems to cause an issue.  

What do I pass in for blank?  Or how do I construct this filter correctly?  

Thanks,

Stizz001