[FIX] gamification: add space in generated query

The generated query in _generate_goals_from_challenge lacked spaces
between the date clause, which caused a traceback.
The traceback only appears since v12 and above, because the dates are
now casted as dates (::date) in SQL, which has to be followed by a
space. Before that it was "fine" as SQL does allow you to write
g.start_date = '10-10-2017'and g.end_date = '11-11-2017'

Closes #28773

closes odoo/odoo#29836

addons/gamification/models/challenge.py

@@ -344,12 +344,12 @@ class Challenge(models.Model):
                 date_clause = ""
                 query_params = [line.id]
                 if start_date:
-                    date_clause += "AND g.start_date = %s"
+                    date_clause += " AND g.start_date = %s"
                     query_params.append(start_date)
                 if end_date:
-                    date_clause += "AND g.end_date = %s"
+                    date_clause += " AND g.end_date = %s"
                     query_params.append(end_date)
-            
+
                 query = """SELECT u.id AS user_id
                              FROM res_users u
                         LEFT JOIN gamification_goal g