[IMP] snailmail: correct default_get behavior

1) Use a default_<fname> context key to give a default value and leave 
the management of the default to the orm level when possible.

2) Do not return unwanted default values in default_get override.

addons/snailmail/static/src/components/message/message_tests.js

@@ -622,7 +622,7 @@ QUnit.test('Missing Required Fields', async function (assert) {
             "action should be the one for missing fields"
         );
         assert.strictEqual(
-            payload.options.additional_context.letter_id,
+            payload.options.additional_context.default_letter_id,
             22, // should be the same as the id returned by search
             "action should have correct letter id"
         );

addons/snailmail/static/src/models/message/message.js

@@ -48,7 +48,7 @@ registerInstancePatchModel('mail.message', 'snailmail/static/src/models/message.
             action: 'snailmail.snailmail_letter_missing_required_fields_action',
             options: {
                 additional_context: {
-                    letter_id: letterIds[0],
+                    default_letter_id: letterIds[0],
                 },
             },
         });

addons/snailmail/wizard/snailmail_letter_missing_required_fields.py

@@ -17,19 +17,19 @@ class SnailmailLetterMissingRequiredFields(models.TransientModel):
 
     @api.model
     def default_get(self, fields):
-        rec = super(SnailmailLetterMissingRequiredFields, self).default_get(fields)
-        letter_id = self.env['snailmail.letter'].browse(self.env.context.get('letter_id'))
-        rec.update({
-            'partner_id': letter_id.partner_id.id,
-            'letter_id': letter_id.id,
-            'street': letter_id.street,
-            'street2': letter_id.street2,
-            'zip': letter_id.zip,
-            'city': letter_id.city,
-            'state_id': letter_id.state_id.id,
-            'country_id': letter_id.country_id.id,
-        })
-        return rec
+        defaults = super(SnailmailLetterMissingRequiredFields, self).default_get(fields)
+        if defaults.get('letter_id'):
+            letter = self.env['snailmail.letter'].browse(defaults.get('letter_id'))
+            defaults.update({
+                'partner_id': letter.partner_id.id,
+                'street': letter.street,
+                'street2': letter.street2,
+                'zip': letter.zip,
+                'city': letter.city,
+                'state_id': letter.state_id.id,
+                'country_id': letter.country_id.id,
+            })
+        return defaults
 
     def update_address_cancel(self):
         self.letter_id.cancel()

addons/snailmail/wizard/snailmail_letter_missing_required_fields_views.xml

@@ -5,6 +5,8 @@
         <field name="model">snailmail.letter.missing.required.fields</field>
         <field name="arch" type="xml">
             <form>
+                <!-- Field present for correct default_get behavior -->
+                <field name="letter_id" invisible="1"/>
                 <p>The customer address is not complete. Update the address here and re-send the letter.</p>
                 <group>
                     <label for="partner_id" string="Address"/>