Here is my experience:
Below bar chart is my slicer. If I select all choices, all 4 quarters appear in bar chart, as expected:
Surprisingly the same happens when none is selected: We know that when we select 1 Qtr only, such as this, then only one Qtr shows:
When I toggle the Q1 button (being the only one selected), so that nothing is selected, the bar chart shows all data - all 4 Qtrs (& blank), as in the second example above.
If several boxes are selected, for example Q1 & Q2 & Q3 in slicer, I first click just one of the selected boxes, e.g., Q3, The bar chart shows just the Q3 data. Then I click on Q3 again, deselecting it, and thus all Qtrs are deselected. The result is in visualizations is as if all slicer boxes were selected, as in "No Qtrs" screen shot above.
That's the say a slicer works from what I can see. A slicer filters based on what you've checked. If you don't check a box, there is no filter applied. The blank appears if you have possible blank choices--or if you have relationships that somehow results in a blank. I usually filter out the blanks on a page to prevent that...but remember if you do that, the ISFILTERED(for that column) will be true. To get around that create a "dummy" duplicate and prevent blanks on that one.