I don't believe this is possible because when choosing Multi-selection in your Datagrid the microflow always expects a list as argument for this microflow (either a selection or all rows, as you can see under ‘Edit on click microflow settings...’). Second part of your question: no, you can not make the visibility dependent on the Datagrid selection.
My question would be if you really need to have this button perform these 2 actions, or could you separate the logic? If you want to create an empty list anyway, you can just create another button, like you suggested, (within or without the Datagrid) and create an empty list in the microflow to begin with.
Wanting two logical operations in one button (one with an argument, and one without), suggests these are actually two different actions and should be separated for maintainability and structure. That's the reason for my follow-up question.