When creating a meeting from a partner/lead, the invitations won't be
created/sent

To reproduce the error:
1. Open a lead
2. Click on Meeting
3. Create a meeting
    - On meeting creation, directly click on "Create", not "Edit"

Error: No invitation has been sent. When editing the created event,
there isn't any invitation on Invitations tab. Same error will happen
when opening the form of a customer instead of a lead (step 1)

For an attendee to be created, the partners associated with the meeting
must be explicitly listed in the creation values:
https://github.com/odoo/odoo/blob/3e20e68f0790a0b0f3b5c4d43f59f235b7d20fef/addons/calendar/models/calendar_event.py#L710-L713


However, in the above use case, the partners identifiers are given
through the context. This explains why the attendees are not created.

OPW-2531496

closes odoo/odoo#74734

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>