Merge branch 'api-empty-return' into 'master'

Gemfile

Gemfile.lock

changelogs/unreleased/api-empty-return.yml

doc/api/README.md

doc/api/award_emoji.md

doc/api/boards.md

doc/api/branches.md

doc/api/broadcast_messages.md

doc/api/build_triggers.md

doc/api/build_variables.md

doc/api/deploy_keys.md

doc/api/enviroments.md

doc/api/issues.md

doc/api/labels.md

doc/api/notes.md

doc/api/runners.md

doc/api/system_hooks.md

doc/api/tags.md

lib/api/api.rb

lib/api/award_emoji.rb