HTTP 500 when navigating to group site

There is a regression from !133641 (merged)

See this ZD ticket:

Reading the ticket, one possible cause is user_id in members is reference an non-existent user.

"exception.class": "ActionView::Template::Error",
  "exception.message": "undefined method `project_bot?' for nil:NilClass",
  "exception.backtrace": [
    "app/models/group.rb:517:in `block (2 levels) in member_owners_excluding_project_bots'",
    "app/models/group.rb:516:in `reject'",
    "app/models/group.rb:516:in `block in member_owners_excluding_project_bots'",
    "app/models/concerns/each_batch.rb:99:in `block (2 levels) in each_batch'",
    "app/models/concerns/each_batch.rb:99:in `block in each_batch'",
    "app/models/concerns/each_batch.rb:69:in `step'",
    "app/models/concerns/each_batch.rb:69:in `each_batch'",
    "app/models/group.rb:515:in `member_owners_excluding_project_bots'",
    "app/models/members/group_member.rb:65:in `last_owner_of_the_group?'",
    "app/policies/group_member_policy.rb:9:in `block in <class:GroupMemberPolicy>'",
    "app/models/ability.rb:89:in `allowed?'",
    "app/controllers/application_controller.rb:242:in `can?'",
    "app/controllers/application_controller.rb:59:in `can?'",
    "app/views/shared/members/_access_request_links.html.haml:3",
    "app/views/groups/_home_panel.html.haml:23",
    "app/views/groups/show.html.haml:24",
    "app/controllers/application_controller.rb:162:in `render'",
    "app/controllers/groups_controller.rb:238:in `render_show_html'",
    "ee/app/controllers/ee/groups_controller.rb:38:in `render_show_html'",
    "app/controllers/groups_controller.rb:104:in `block (2 levels) in show'",
    "app/controllers/groups_controller.rb:99:in `show'",
    "ee/lib/gitlab/ip_address_state.rb:10:in `with'",
    "ee/app/controllers/ee/application_controller.rb:45:in `set_current_ip_address'",
    "app/controllers/application_controller.rb:498:in `set_current_admin'",
    "lib/gitlab/session.rb:11:in `with_session'",
    "app/controllers/application_controller.rb:489:in `set_session_storage'",
    "lib/gitlab/i18n.rb:114:in `with_locale'",
    "lib/gitlab/i18n.rb:120:in `with_user_locale'",
    "app/controllers/application_controller.rb:480:in `set_locale'",
    "app/controllers/application_controller.rb:473:in `set_current_context'",
    "lib/gitlab/metrics/elasticsearch_rack_middleware.rb:16:in `call'",
    "lib/gitlab/middleware/memory_report.rb:13:in `call'",
    "lib/gitlab/middleware/speedscope.rb:13:in `call'",
    "lib/gitlab/database/load_balancing/rack_middleware.rb:23:in `call'",
    "lib/gitlab/middleware/rails_queue_duration.rb:33:in `call'",
    "lib/gitlab/etag_caching/middleware.rb:21:in `call'",
    "lib/gitlab/metrics/rack_middleware.rb:16:in `block in call'",
    "lib/gitlab/metrics/web_transaction.rb:46:in `run'",
    "lib/gitlab/metrics/rack_middleware.rb:16:in `call'",
    "lib/gitlab/jira/middleware.rb:19:in `call'",
    "lib/gitlab/middleware/go.rb:20:in `call'",
    "lib/gitlab/middleware/query_analyzer.rb:11:in `block in call'",
    "lib/gitlab/database/query_analyzer.rb:37:in `within'",
    "lib/gitlab/middleware/query_analyzer.rb:11:in `call'",
    "lib/gitlab/middleware/multipart.rb:173:in `call'",
    "lib/gitlab/middleware/read_only/controller.rb:50:in `call'",
    "lib/gitlab/middleware/read_only.rb:18:in `call'",
    "lib/gitlab/middleware/same_site_cookies.rb:27:in `call'",
    "lib/gitlab/middleware/path_traversal_check.rb:25:in `call'",
    "lib/gitlab/middleware/handle_malformed_strings.rb:21:in `call'",
    "lib/gitlab/middleware/basic_health_check.rb:25:in `call'",
    "lib/gitlab/middleware/handle_ip_spoof_attack_error.rb:25:in `call'",
    "lib/gitlab/middleware/request_context.rb:15:in `call'",
    "lib/gitlab/middleware/webhook_recursion_detection.rb:15:in `call'",
    "config/initializers/fix_local_cache_middleware.rb:11:in `call'",
    "lib/gitlab/middleware/compressed_json.rb:44:in `call'",
    "lib/gitlab/middleware/rack_multipart_tempfile_factory.rb:19:in `call'",
    "lib/gitlab/middleware/sidekiq_web_static.rb:20:in `call'",
    "lib/gitlab/metrics/requests_rack_middleware.rb:79:in `call'",
    "lib/gitlab/middleware/release_env.rb:13:in `call'"
  ],

/cc @alvin