Don't ignore the "Done" message of `dein#check_update()` even if there are no updated plugins

[kg8m]

Hi. Thanks for developing dein.vim. No dein.vim, no my Vim life.

I have an issue that the "Done" message, e.g., "Done: (2023/01/02 03:04:05)", of dein#check_update() is ignored if there are no updated plugins. This issue is caused as follows:

1. dein#check_update() calls dein#install#_check_update() internally
2. dein#install#_check_update() calls dein#install#_get_updated_plugins() and checks if there are updated plugins
3. If there are no updated plugins, dein.vim calls s:notify() for sending the "Done" message and finishes its update process
4. Because the variable context is empty, s:notify() does nothing and the "Done" message is ignored
   • The context is derived from s:global_context, that is empty because dein#install#_get_updated_plugins() makes it empty

This pull request fixes the issue. Even if context is empty, s:notify() will call s:updates_log() and assign s:progress. Then only if context isn't empty, s:notify() will check context.message_type and call dein#util#_notify().

[Shougo]

Thank you. Merged.

[kg8m]

Thank you!