avoid interpretation of fixed costs

Author: kopsha

src/microinvoicer/templates/pdf_time_invoice_template.html

@@ -186,7 +186,7 @@ <h2>{{ invoice_title }}</h2>
                     <td class="unit"></td>
                     <td></td>
                     <td>{{ invoice_attached_cost }}</td>
-                    <td></td>
+                    <td>--</td>
                 </tr>
                 {% endif %}
                 <tr class="overline bold-text">