[Workpad Loader] Redirect to home if there are no remaining workpads …

…left after delete (elastic#682)

Closes https://github.com/elastic/kibana-canvas/issues/674

# Before
Deleting the last workpad in the set would not clear it out of the area, allowing you to continue making changes. Attempts to persist updates would result in a 404
![wkpddeletebefore](https://user-images.githubusercontent.com/908371/41438488-42fb1684-6fdc-11e8-8b66-5de6f5d647e3.gif)

# After
Delete the last workpad, and you'll be taken to the home route
![wkpddeleteafter](https://user-images.githubusercontent.com/908371/41438506-519f9a02-6fdc-11e8-8910-7f0e74fbd431.gif)

public/components/workpad_loader/index.js

@@ -62,11 +62,13 @@ export const WorkpadLoader = compose(
         const nextWorkpad = workpadState.workpads[0];
         if (nextWorkpad != null) {
           props.router.navigateTo('loadWorkpad', { id: nextWorkpad.id, page: 1 });
+
+          // update the workpad list, filtering out the removed workpad
+          setWorkpads(workpadState);
+        } else {
+          props.router.navigateTo('home');
         }
       }
-
-      // update the workpad list, filtering out the removed workpad
-      setWorkpads(workpadState);
     },
   })
 )(Component);