Do not fail when query stats are missing
For for self-managed only migrations, we end up with not query statistics collected, as there are no queries, for example
{"version":20230811150636,"name":"SwapCommitUserMentionsNoteIdToBigintForSelfManaged","walltime":2.5833030939102173,"success":true,"total_database_size_change":0,"error_message":null,"query_statistics":null}
This is causing the notifier job to fail with undefined method
map' for nil:NilClass (NoMethodError)` - https://ops.gitlab.net/gitlab-com/database-team/gitlab-com-database-testing/-/jobs/10944753.
This MR is fixing this. Alternatively we can change the other end to set missing statistic as []
instaed of nil
, but fixing it here seems easier.