Warning about costs not in newsletter if event price = 0

One-sentence description

The warning about the costs and registration/un-registration deadline is not shown when the price of the event is 0.

Current behaviour / Reproducing the bug

1. Add an event without a price
2. The 'Show warnings about costs' checkbox is checked
3. Show preview of the newsletter
4. The warning is not in the newsletter

Expected behaviour

The warning should be there if the checkbox is checked