Setting `singleuser.storage.extraVolumes` and `extraVolumeMounts` as arrays in subchart, fails

[GeorgianaElena]

Bug description

#3690 introduced the option to define singleuser.storage.extraVolumes and extraVolumeMounts as dicts, while also allowing it to be an array according to the schema.

However, in practice, when I tried to upgrade a hub from z2jh 4.0.0 to 4.3.1, without updating singleuser.storage.extraVolumes to a map, I saw a bunch of helm warnings about it because of the type mismatch.

This is because z2jh values.yaml sets the default value to be {}, so only dicts will be accepted, otherwise, even though the upgrade happens, the configuration just gets discarded if it's set as an array I beleive.

coalesce.go:223: warning: destination for basehub.jupyterhub.singleuser.storage.extraVolumeMounts is a table. Ignoring non-table value ([map[mountPath:/home/jovyan/shared name:home readOnly:true subPath:_shared] map[mountPath:/dev/shm name:dev-shm] map[mountPath:/home/rstudio name:home subPath:{escaped_username}] map[mountPath:/home/rstudio/shared name:home readOnly:true subPath:_shared]])
coalesce.go:223: warning: destination for basehub.jupyterhub.singleuser.storage.extraVolumes is a table. Ignoring non-table value ([map[emptyDir:map[medium:Memory] name:dev-shm]])
coalesce.go:220: warning: cannot overwrite table with non table for basehub.jupyterhub.singleuser.storage.extraVolumeMounts (map[])
coalesce.go:220: warning: cannot overwrite table with non table for basehub.jupyterhub.singleuser.storage.extraVolumes (map[])
coalesce.go:223: warning: destination for basehub.jupyterhub.singleuser.storage.extraVolumeMounts is a table. Ignoring non-table value ([map[mountPath:/home/jovyan/shared name:home readOnly:true subPath:_shared] map[mountPath:/dev/shm name:dev-shm] map[mountPath:/home/rstudio name:home subPath:{escaped_username}] map[mountPath:/home/rstudio/shared name:home readOnly:true subPath:_shared]])
coalesce.go:223: warning: destination for basehub.jupyterhub.singleuser.storage.extraVolumes is a table. Ignoring non-table value ([map[emptyDir:map[medium:Memory] name:dev-shm]])