Destroying a project causes post_decline_request to be executed

/cc: @rymai

https://sentry.gitlap.com/gitlab/gitlabcom/issues/7403/

NoMethodError: undefined method `id' for nil:NilClass
  app/services/notification_service.rb:182:in `decline_project_access_request'
    mailer.member_access_denied_email(project_member.real_source_type, project_member.project.id, project_member.user.id).deliver_later
  lib/gitlab/metrics/instrumentation.rb:156:in `decline_project_access_request'
    retval    = super
  app/models/members/project_member.rb:168:in `post_decline_request'
    notification_service.decline_project_access_request(self)
  app/services/projects/destroy_service.rb:24:in `block in execute'
    project.destroy!
  app/services/projects/destroy_service.rb:23:in `execute'
    Project.transaction do
...
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