🐛(backend) fix KeyError crash when role is undefined in request data

Use dict.get() instead of direct key access to prevent server crashes when
role field is missing. Fix inherited from magnify project codebase.

Author: lebaudantoine

src/backend/core/api/serializers.py

@@ -37,7 +37,7 @@ def validate(self, data):
             # Update
             self.instance
             and (
-                data["role"] == models.RoleChoices.OWNER
+                data.get("role") == models.RoleChoices.OWNER
                 and not self.instance.resource.is_owner(user)
                 or self.instance.role == models.RoleChoices.OWNER
                 and not self.instance.user == user