FIX: StaticController#enter should not redirect to invalid paths (#27913)

This commit updates `StaticController#enter` to not redirect to invalid
paths when the `redirect` param is set. Instead it should redirect to `/` when the
`redirect` param is invalid.

spec/requests/static_controller_spec.rb

@@ -290,7 +290,7 @@ RSpec.describe StaticController do
       end
     end
 
-    context "with a full url to someone else" do
+    context "with a full url to an external host" do
       it "redirects to the root path" do
         post "/login.json", params: { redirect: "http://eviltrout.com/foo" }
         expect(response).to redirect_to("/")
@@ -320,6 +320,13 @@ RSpec.describe StaticController do
         expect(response).to redirect_to("/")
       end
     end
+
+    context "when the redirect path is invalid" do
+      it "redirects to the root URL" do
+        post "/login.json", params: { redirect: "test" }
+        expect(response).to redirect_to("/")
+      end
+    end
   end
 
   describe "#service_worker_asset" do