Jetpack_Sync_Module_Options::get_all_options() returns non-existing wp_options

If an option doesn't exist in the wp_options table, it'll still be sent (as an empty value) in the full_sync action.

To reproduce:

- Add a non-existing option to the whitelist, or delete an existing option
- Run a full sync
- Observe the option being sent

Start by tracing Jetpack_Sync_Module_Options::get_all_options and the full_sync action, then reproduce the case with a whitelisted option missing from wp_options. Done means missing options are omitted from the full_sync payload rather than sent with an empty value.

Written by the indexing model from the issue text.