plugin: hooks: add explicit returns

plugin: hooks: add explicit returns

to avoid returning something unexpected. Finish what
afeda182566292be15413d9b874720876eac14c9 already started for all the other
plugins. At least for ZFS's on_add_hook this is necessary (adding a ZFS storage
currently fails as reported here [0]), but it cannot hurt
in the other places either as the only hooks we expect to return something
currently are PBS's on_add_hook and on_update_hook.

[0]: https://forum.proxmox.com/threads/gui-add-zfs-storage-verification-failed-400-config-type-check-object-failed.79734/

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>