Clear tags when no tags are given

app/controllers/site_problems_controller.rb

@@ -62,7 +62,7 @@ def edit_tags
 
   def update_tags
     site_problem = SiteProblem.find_or_create_by(site: site, problem_id: params[:problem_id])
-    site_problem.tags = params[:tags].map { |tag| Tag.find_by!(name: tag) }
+    site_problem.tags = (params[:tags] || []).map { |tag| Tag.find_by!(name: tag) }
     site_problem.save!
     Activity.create(user: @current_user, target: site_problem, kind: :tags_update, parameters: site_problem.tags.pluck(:id))
     redirect_to send(:"#{site}_path", params[:problem_id])

spec/support/shared_examples/site_problems_controller.rb

@@ -124,17 +124,27 @@ def create_wa(problem, user, submitted_at)
     end
 
     context 'with sign-in' do
+      include SiteProblemsHelper
+
       before do
         session[:user_id] = user.id
       end
 
       it 'works' do
-        extend SiteProblemsHelper
         post :update_tags, problem_id: problem1.problem_id, tags: [tag1.name, tag2.name]
         expect(response).to redirect_to(problem_path(problem1))
         problem1.reload
         expect(problem1.tags.map(&:name)).to match_array([tag2.name, tag1.name])
       end
+
+      context 'when no tags are given' do
+        it 'clears tags' do
+          post :update_tags, problem_id: problem1.problem_id
+          expect(response).to redirect_to(problem_path(problem1))
+          problem1.reload
+          expect(problem1.tags).to be_empty
+        end
+      end
     end
   end
 end