[API] Fix creating issue when assignee_id is empty

See https://gitlab.com/gitlab-org/gitlab-ce/issues/42025

changelogs/unreleased-ee/42025-fix-issue-api.yml

+---
+title: "[API] Fix creating issue when assignee_id is empty"
+merge_request:
+author:
+type: fixed

lib/api/helpers/common_helpers.rb

@@ -3,8 +3,10 @@ module API
     module CommonHelpers
       def convert_parameters_from_legacy_format(params)
         params.tap do |params|
-          if params[:assignee_id].present?
-            params[:assignee_ids] = [params.delete(:assignee_id)]
+          assignee_id = params.delete(:assignee_id)
+
+          if assignee_id.present?
+            params[:assignee_ids] = [assignee_id]
           end
         end
       end

spec/requests/api/issues_spec.rb

@@ -851,6 +851,15 @@ describe API::Issues, :mailer do
         expect(json_response['assignee']['name']).to eq(user2.name)
         expect(json_response['assignees'].first['name']).to eq(user2.name)
       end
+
+      it 'creates a new project issue when assignee_id is empty' do
+        post api("/projects/#{project.id}/issues", user),
+          title: 'new issue', assignee_id: ''
+
+        expect(response).to have_gitlab_http_status(201)
+        expect(json_response['title']).to eq('new issue')
+        expect(json_response['assignee']).to be_nil
+      end
     end
 
     context 'single assignee restrictions' do