[patch] Only migrate found collections (GSI-1308)

[TheByronHimes]

This PR fixes an issue found in testing where uninitialized collections crashed the migration process (can't rename a collection that doesn't exist). The migration process previously assumed the collections would exist, but now it will check before staging/unstaging and do nothing if they don't exist. This will allow migration-equipped services to start up in test environments that might not have test data already created.

Of course, when testing the migrations specifically in those environments, we need to have test data present before starting the services. That's a team coordination thing though, imo and not an app/config problem.