When you combine a retrieve and an aggregate count mendix will optimize these two actions into a COUNT query against the database, so you should be carefull if you plan to use the list elements afterwards.
You can also use
$MyEntityList != empty
to check if the list has 0 elements.
You need an aggregate list activity to count the list
You could try to do this in a microflow.
Retrieve objects / Range: All / Output List /
Make a split and just say
$Objectlist = empty
And save it
Then make the true and false statements you want.
If you also need the number you could do a Count between the Retrieve and Split.
This works for me atleast... dont know if its the best option