500 response on GET custom attributes if project is not found

Summary

If a project is deleted, GET to /api/v4/projects/:id/custom_attributes returns 500

Steps to reproduce

1. use admin token to send a GET request to /api/v4/projects/:id/custom_attributes with an invalid id

Example Project

What is the current bug behavior?

What is the expected correct behavior?

Relevant logs and/or screenshots

"route":"/api/:version/groups/:id/custom_attributes",
"exception.class":"NoMethodError","exception.message":"undefined method 
`custom_attributes' for nil:NilClass",
"exception.backtrace":["lib/api/custom_attributes_endpoints.rb:26:
in `block (2 levels) in <module:CustomAttributesEndpoints>'","lib/api/api_guard.rb:213:
in `call'","lib/gitlab/middleware/speedscope.rb:13:in `call'","lib/gitlab/request_profiler/middleware.rb:17:in `call'","lib/gitlab/jira/middleware.rb:19:
in `call'","lib/gitlab/middleware/go.rb:20:in 

Output of checks

Results of GitLab environment info

self hosted gitlab-ce v14.0.8

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

https://gitlab.com/yuanchenlu/gitlab-rails/-/merge_requests/1/diffs