Error in notifier job: notifier/migration.rb:201:in `init_queries': undefined method `map' for nil:NilClass (NoMethodError)

In gitlab-org/gitlab!129464 (comment 1518187843) , we got an error in the notifier job of the pipeline (https://ops.gitlab.net/gitlab-com/database-team/gitlab-com-database-testing/-/pipelines/2227220). The notifier shouldn't ever fail, so we should find out what went wrong and fix the bug.

/home/gitlab-runner/builds/dGxsBu4n/0/gitlab-com/database-team/gitlab-com-database-testing/notifier/migration.rb:201:in `init_queries': undefined method `map' for nil:NilClass (NoMethodError)
	from /home/gitlab-runner/builds/dGxsBu4n/0/gitlab-com/database-team/gitlab-com-database-testing/notifier/migration.rb:197:in `init_stats'
	from /home/gitlab-runner/builds/dGxsBu4n/0/gitlab-com/database-team/gitlab-com-database-testing/notifier/migration.rb:72:in `initialize'
	from /home/gitlab-runner/builds/dGxsBu4n/0/gitlab-com/database-team/gitlab-com-database-testing/notifier/migration.rb:55:in `new'
	from /home/gitlab-runner/builds/dGxsBu4n/0/gitlab-com/database-team/gitlab-com-database-testing/notifier/migration.rb:55:in `from_directory'
	from /home/gitlab-runner/builds/dGxsBu4n/0/gitlab-com/database-team/gitlab-com-database-testing/notifier/result.rb:15:in `block in from_directory'
	from /home/gitlab-runner/builds/dGxsBu4n/0/gitlab-com/database-team/gitlab-com-database-testing/notifier/result.rb:15:in `map'
	from /home/gitlab-runner/builds/dGxsBu4n/0/gitlab-com/database-team/gitlab-com-database-testing/notifier/result.rb:15:in `from_directory'
	from /home/gitlab-runner/builds/dGxsBu4n/0/gitlab-com/database-team/gitlab-com-database-testing/notifier/multi_db_result.rb:13:in `block in from_directory'
	from /home/gitlab-runner/builds/dGxsBu4n/0/gitlab-com/database-team/gitlab-com-database-testing/notifier/multi_db_result.rb:12:in `transform_values'
	from /home/gitlab-runner/builds/dGxsBu4n/0/gitlab-com/database-team/gitlab-com-database-testing/notifier/multi_db_result.rb:12:in `from_directory'
	from notifier.rb:99:in `feedback_for'
	from notifier.rb:92:in `comment_for'
	from notifier.rb:29:in `send'
	from /usr/lib/ruby/vendor_ruby/thor/command.rb:27:in `run'
	from /usr/lib/ruby/vendor_ruby/thor/invocation.rb:126:in `invoke_command'
	from /usr/lib/ruby/vendor_ruby/thor.rb:369:in `dispatch'
	from /usr/lib/ruby/vendor_ruby/thor/base.rb:444:in `start'
	from notifier.rb:113:in `<main>'