feat(wm): drop empty containers on ws update

We shouldn't ever have empty containers, but never say never because
someone on the Discord has an empty container with no Windows that
continues to take up a tile. This commit adds a call to drop all
containers without any windows whenever Workspace::update is called.

komorebi/src/workspace.rs

@@ -480,6 +480,9 @@ impl Workspace {
             return Ok(());
         }
 
+        // make sure we are never holding on to empty containers
+        self.containers_mut().retain(|c| !c.windows().is_empty());
+
         let container_padding = self
             .container_padding()
             .or(self.globals().container_padding)