I am unfamiliar with the Python Fuel SDK, so my first suggestion could be completely dumb due to my ignorance.
Can you use the simpleoperator "isNotNull" equivalent in python (if it exists)?
This next answer is not exactly the most elegant solution, but you could try:
1 Pull the rows (without filters)
2 Take a rowcount of the data and start a for loop
3 For loop should be something like: (using ampscript as example)
FOR @i = 1 TO @rowcount DO
SET @row = Row(@yourdata,@i)
SET @length = length(field(@row,'Fieldname'))
IF @length > 0 THEN
ADD to a new object/array or DE, whichever you prefer
Another option to use instead of Length() is to use the IsNull() or EMPTY() function instead.
4 Use the new DE or object/array for your need.
This will definitely add time to your scripts, so on a large file it will not work - but as I do not see a simple alternative via the filter function I thought I should share.
A final option, very much duct tape and gum, is to build a filter activity that uses the argument "Fieldname is empty" and call that activity as your filter. The issue is that it will only work on a single Data Extension and cannot be used universally or on non-data extensions.