[FIX] mail: add new note without date from activity menu

Before this commit, it was not possible to add a note
from the activity menu when no datetime has been selected
from the datepicker.

This is due to the server-side code that requires a deadline
date to be set, otherwise no note is created. Even though
the placeholder of the datetime tells 'Today', no datetime
was provided to the server, thus the problem mentionned above.

This commit fixes the issue by ensuring that a deadline date
is always provided to the server from the `/note/new` route.
In particular, if no datetime has been selected, it still
displays 'Today' and sets the current datetime before the
RPC.

Task-ID 1875668