[FIX] sms: avoid crashing when no res_model of res_ids

sms_composer could crash in case you didn't pass it the active_id,
active_ids context keys or don't call the default_get

In that case it would try to access `self.env[False]` which would crash
in the `_compute_recipients_count`

We also have to ensure that we have a default value for `self.res_ids`
because `literal_eval(False)` will crash

closes odoo/odoo#38461

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