[io] FilterSource: make use of the new EmptySource eclass, over `Buffer("")`
[io] FilterSource: make use of the new EmptySource eclass, over `Buffer("")`

xzero/x0,christianparpart/x0,christianparpart/x0,christianparpart/x0,xzero/x0,xzero/x0,christianparpart/x0,xzero/x0,xzero/x0,xzero/x0,christianparpart/x0,christianparpart/x0,christianparpart/x0