Empty board and committee pages not consistent

One-sentence description

Going to the board page without any boards is not handled the same as going to the committee page without any committees.

Current behaviour

If there are no boards, going to the board page results in a 404 page because of an error (No Board matches the given query.). Going to the committee page without any committees however shows a nice message "There are no committees!".

Looking at the template for the board page it looks like a similar message should be shown:

          {% empty %}
          {% trans 'There are no boards!' %}
          {% endfor %}

Expected behaviour

If there are no boards, going to the board page results in There are no boards!.

Steps to reproduce

1. Go to the boards page without any boards.