bans: fix exception when username is blank.

Fix exception when submitting the ban form and the username is blank.

app/models/ban.rb

@@ -43,7 +43,7 @@ class Ban < ApplicationRecord
   end
 
   def validate_user_is_bannable
-    errors.add(:user, "is already banned") if user.is_banned?
+    errors.add(:user, "is already banned") if user&.is_banned?
   end
 
   def update_user_on_create

test/functional/bans_controller_test.rb

@@ -100,6 +100,11 @@ class BansControllerTest < ActionDispatch::IntegrationTest
           assert_response :success
         end
       end
+
+      should "not raise an exception on a blank username" do
+        post_auth bans_path, @mod, params: {}
+        assert_response :success
+      end
     end
 
     context "update action" do