Verify project id in API

* Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/351520
* Sentry error:
https://sentry.gitlab.net/gitlab/gitlabcom/issues/3184048

**Problem**

It is possible to skip Grape validation for id for http OPTIONS
method.

**Solution**

Verify that id is present

Changelog: fixed

lib/api/helpers.rb

@@ -117,6 +117,8 @@ module API
 
     # rubocop: disable CodeReuse/ActiveRecord
     def find_project(id)
+      return unless id
+
       projects = Project.without_deleted
 
       if id.is_a?(Integer) || id =~ /^\d+$/

spec/lib/api/helpers_spec.rb

@@ -76,6 +76,12 @@ RSpec.describe API::Helpers do
           expect(subject.find_project(non_existing_id)).to be_nil
         end
       end
+
+      context 'when project id is not provided' do
+        it 'returns nil' do
+          expect(subject.find_project(nil)).to be_nil
+        end
+      end
     end
 
     context 'when ID is used as an argument' do