Implement Empty widget

It can be useful if you want to fill some space with nothing.

wgpu/src/renderer/widget.rs

@@ -1,6 +1,7 @@
 mod button;
 mod checkbox;
 mod column;
+mod empty;
 mod image;
 mod radio;
 mod row;

wgpu/src/renderer/widget/empty.rs

@@ -0,0 +1,8 @@
+use crate::{Primitive, Renderer};
+use iced_native::{empty, MouseCursor, Rectangle};
+
+impl empty::Renderer for Renderer {
+    fn draw(&mut self, _bounds: Rectangle) -> Self::Output {
+        (Primitive::None, MouseCursor::OutOfBounds)
+    }
+}