Hotfix: Don't assume subscription has a note when showing `admin/invoices/failed/`

[chrisjsimpson]

Is your feature request related to a problem? Please describe.

UndefinedError
jinja2.exceptions.UndefinedError: 'None' has no attribute 'strip'

https://github.com/Subscribie/subscribie/blob/51fd8052aa08d7fd3dff157fcb9cfafe0f61ed6d/subscribie/blueprints/admin/templates/admin/invoice/failed_invoices.html

                            {% for invoice in debtor['failedInvoices'] %}
                            <tr>
                              <td>{{ invoice.subscribie_subscription.plan.title }}
                              </td>
                              <td>{% if invoice.subscribie_subscription.note %}
                                    {% if invoice.subscribie_subscription.note.note.strip() != '' %}
                                      {{ invoice.subscribie_subscription.note.note }}
                                    {% else %}
                                        <small style="color: lightgrey">None</small>
                                    {% endif %}
                                  {% endif %}

Describe the solution you'd like

Describe alternatives you've considered
actually use a type strict templating language.

Additional context