Only strip brief supplier question and answer if they're strings
Only strip brief supplier question and answer if they're strings

Content loader replaces empty strings with `None` values when
sending the request to the API, which makes the `.strip()` call
fail.

Instead, we move the logic to model field validators and only call
strip if the field value is of string type.

With an upcoming change to the buyer frontend the fields will be
stripped before being sent to the API, so we could potentially get
rid of the `.strip` call completely (similar to how we don't strip
whitespace from other direct database fields), but it feels like
the API should be responsible for this.

alphagov/digitalmarketplace-api,alphagov/digitalmarketplace-api,alphagov/digitalmarketplace-api