[FIX] survey_record_generation: questions without answer no longer empty fields

When a question was not displayed (conditional question) or skipped, its
field was still given to create() with a None value. The field was then
emptied instead of keeping its default or computed value.

Since the link to an already created record is given at creation, this
also made sale.subscription.line price_unit equal to 0 when the "free
amount" question was not displayed, instead of the product price.

Fields without value are now left out of the values, which also allows
several conditional questions to fill up the same field (e.g. one list
of amounts per periodicity).

Assisted-By: Claude Opus 5.5 <noreply@anthropic.com>

survey_record_generation/models/survey_user_input.py

@@ -61,6 +61,11 @@ class SurveyUserInput(models.Model):
                     )
                     field_name: str = field_value.field_id.name
 
+                    if value is None:
+                        # No value (e.g. question not displayed or skipped):
+                        # keep the default/computed value of the field, and
+                        # let another question fill up the same field.
+                        continue
                     vals[field_name] = value
 
                 existing_record = self.find_existing_record(record_creation, vals)
@@ -195,7 +200,7 @@ class SurveyUserInput(models.Model):
         if unique_fields:
             uniq_domain = []
             for uniq_field in unique_fields:
-                uniq_domain.append((uniq_field, "=", vals[uniq_field]))
+                uniq_domain.append((uniq_field, "=", vals.get(uniq_field, False)))
             duplicate = self.env[model].search(uniq_domain, limit=1)
         return duplicate