In the Sprout circuit, zero-valued input notes can bypass the check against the anchor. These are so-called "dummy notes" as they don't affect the balance equation.
We need this in Sapling too, so that input arity can be hidden. Otherwise, it's a privacy regression from Sprout, strictly speaking. (Output arity can be hidden already; you can cheaply create zero-valued output notes.)
In the Sprout circuit, zero-valued input notes can bypass the check against the anchor. These are so-called "dummy notes" as they don't affect the balance equation.
We need this in Sapling too, so that input arity can be hidden. Otherwise, it's a privacy regression from Sprout, strictly speaking. (Output arity can be hidden already; you can cheaply create zero-valued output notes.)